Prepare the Swiss chard by cleaning it well (see note), cutting off the stems and chopping them into 3/4 - 1 & 1/2 -inch pieces, then cutting the leaves into thin slices or small pieces. The total should be about 2 cups of stems and 6-8 heaping cups of leaves.
Heat the olive oil. Add the sliced onion on medium heat and sauté for approximately 5 minutes until the onion is transparent.
Add the stems, coriander and Aleppo or crushed red pepper, stir to combine, cover and cook for 3-4 minutes on low heat.
Add the Swiss chard leaves, stir, tossing the mixture so that the olive oil and semi-cooked stems and onion thoroughly combine with the uncooked leaves. Then cover again, this time cooking for an additional 4-6 minutes until the leaves are thoroughly wilted and taste fully cooked.
Sprinkle the balsamic vinegar over the vegetables and add salt to taste.
In order to clean the Swiss chard, either soak the leaves in a large pot of water or rinse them well in a colander. In either event, dry them before separating the stems and cutting both the stems and leaves.
I prefer Aleppo pepper to crushed red pepper in this recipe because the former goes so well with coriander. But given the times we're in, if all you have are red pepper flakes, that's fine.
Roasting brings out the flavor of the coriander nicely. I bought already roasted ground coriander. You could, of course, roast your own seeds in a small, dry cast iron pan and then crush them in a spice grinder or with a mortar and pestle.