I seem to be into monster vegetables this month, which is how I got to these stuffed portobello mushroom caps.

Besides my humongous carrots (turned into crispy baked chips) from the CSA, I had two packages of gigantic mushroom caps from Costco taking up half of a shelf in my refrigerator. The first package got stuffed with kale and topped with cheese and matzo-meal, a Passover-friendly offering for my vegetarian seder guests.

Once the seder was over, 4 mushrooms remained. They may not have been the size of baseball caps, but they were almost usable as edible yarmulkes (kippot). I do love Costco, the source for too-much-of-everything, but what to do with these mushroom caps? 

As I had been craving macaroni and cheese, I decided they were ideal vehicles for a pasta-less cheese sauce dish. I envisioned these stuffed portobello mushroom caps with the mushroom as platter, holding onto – but barely – a creamy cheese sauce and topped with a crunchy top. Of course, mushrooms do go so well with greens, including spinach, so I added a layer of greens just above the mushroom cap that merged into the cheese sauce. 

Messy? You bet. Delicious. Absolutely. 

They are substantial enough for a vegetarian main course, but could also work as a side dish, particularly if you use smaller mushrooms or cut them in half. If you use smaller mushroom caps, there will be less scraping out of the gills that are more prominent in the larger caps, but the smaller ones are more difficult to fill. Given that trade-off, my preference is for caps that are at least 3-4 inches wide. 

LIke my earlier version of stuffed mushrooms (featuring spinach mixed with ricotta, and stuffed in a much more genteel-looking mushroom cap), you can bake these stuffed portobello mushroom caps in a toaster or “regular” oven. If you don’t like mess, put them in a foil lined pan with sides and simply serve them as a casserole.

Cheesy Greens-Stuffed Portobello Mushrooms

Servings – 4 as a main course, 6-8 as a side dish      Cost – $10 (quite variable due to mushrooms)

Ingredients

• 4 large or more portobello mushroom caps
• 5 tablespoons/½ stick+ 1 tablespoon butter
• ¼ cup (4 tablespoons) flour
• 2 cups milk (I use non-fat)
• 2 cups grated cheese, well-packed (1½ cups for sauce, ½ cup for topping) – preferably Monterey Jack or other cheese that melts well, such as mozzarella
• 1 teaspoon Dijon mustard
• ½ teaspoon Worcestershire sauce
• ¼ – ½ teaspoon kosher or sea salt and freshly ground pepper
• ½ cup chopped onion (about ½ medium onion)
• 4 cups chopped greens (could use all spinach or any combination of spinach, and others such as Asian broccoli-rapini, or baby kale)
• ½ cup chopped tomatoes (optional) – if using whole tomatoes, peel and seed them
• ½ cup homemade breadcrumbs or Panko (matzo meal works too)

Equipment 

• Cutting board
• 2 knives – 1 medium-large and 1 small 
• 2 spoons – 1 large and 1 small (grapefruit spoon with serrated edge works well)
• Measuring cups (for liquids and solids)
• Measuring spooons
• 2 bowls, 1 medium-large and 1 small
• Large pan
• Large spoon
• Whisk
• Spatula
• Foil lined, oiled or buttered oven-safe pan (I tried it both with and without the foil – the latter is much easier on clean-up)

Preparation

• Preheat the oven to 375 degrees F. 
• Hollow out the (washed and dried) mushrooms by cutting off the thick stem and gently scooping out the dark gills with a small spoon. Set aside the gills and gently cut any thick parts of the mushroom, so that the cap is not more than about ½ inch thick.  Discard the gills and keep the rest that was cut away, chopping it roughly into small-medium size pieces. There is no need to be too precise – a bit of the dark gills can remain and the cap need not be uniformly thick, as long as you cut down the thickest parts. 

• Mix ½ cup of the grated cheese and the breadcrumbs/Panko/matzo meal and set aside. 
• Melt 1 tablespoon of butter. Sauté the chopped onionsfor 3-4 minutes until golden brown. Add the chopped mushroom bits and cook them for a few minutes until they render their juices. Then add the greens, cooking them for a few minutes until they wilt. If the mixture is too dry, add a tablespoon or two of water to help the greens along. Add ¼ teaspoon of salt and a bit of pepper. Remove the mixture to a bowl and set it aside. Wipe the pan clean or rinse it out and dry it well.

• Now for the roux, a fancy French term for the base for cream sauce. It is basically equal parts melted butter and flour, cooked together with milk or cream added slowly to bring it to a saucy consistency. It’s not difficult to make if you are patient – don’t rush it and keep stirring. My “trick is to preheat the milk, which both speeds up and eases the process of smoothing out the roux. I microwave the milk in a microwave-safe measuring cup for 2 minutes on high, but check your own microwave. It should be quite warm, but not boiling. 
• Melt the remainder of the butter and add the flour, whisking it until it is completely incorporated. Continue whisking the mixture for about 2 minutes to cook the flour, then slowly add the milk, while continuing to whisk the mixture. The mixture may clump initially, but if you add the milk slowly and whisk it continuously, it will smooth out. Keep cooking the sauce until it thickens, then, add the mustard and Worcestershire sauce, a few twists of freshly ground pepper and ¼ teaspoon of salt if the cheese isn’t too salty. Finally add the remaining 1½ cup of cheese and mix it in until the sauce is again smooth. With the cheese added, the consistency becomes almost elastic. 

• Place the mushroom caps in the pan, spoon the greens mixture into the mushroom caps, add the chopped tomatoes (if you’re using them), and spoon the cheese sauce over the greens. Add the cheese and breadcrumb/panko/matzo meal topping and bake for about 20 minutes until the sauce is bubbly and the top is browned.

By the way, if you’re wondering why I changed the spelling from “portabella” (in the previous recipe for stuffed mushrooms) to “portobello” in this one – it’s simple. I looked up the name of these large brown mushrooms and it turns out that the latter is currently the preferred spelling. So, I’m changing with the times. 

In any event, stuffed portobello mushroom caps are a healthy and satisfying main course no matter how you spell their name.