A great way to use a large zucchini without messing up your whole kitchen.
Preheat the oven to 400 degrees F.
Hasselback the zucchini into 1/2" slices. That means cutting it, but not all the way through, using two wooden spoons to guide the knife so it doesn't hit the cutting board.
Place the zucchini on a large piece of foil in a roasting pan. Using the pastry brush, brush the slices and the top with olive oil. Loosely close the foil and bake the zucchini for about 25 minutes until it is mostly cooked but not mushy.
While the zucchini bakes, slice the mozzarella, tomatoes, and onion. Put them in sets sprinkled with the salt, pepper, garlic powder and herbs along the cutting board so they are ready to add at the next stage.
Open the foil and let the zucchini cool until you can handle it. Push the sliced cheese, tomato, and onion in between the zucchini slices. Add grated cheese on top and close the foil over the zucchini. Return the now stuffed zucchini to the oven and bake for about another 20 minutes until the cheese begins to melt. If desired, open the foil and broil the top for another 1-2 minutes to brown the grated cheese.
I've specified the quantities I used for a 2 lb/900 g chunk of a mammoth zucchini. You can use this technique for a single zucchini, a chunk of a mammoth one, or 2 smaller ones. In any event, keep the slices of vegetables and mozzarella proportional to the size of the zucchini.