In a frying pan melt butter, add diced onions. Cook for about 20 minutes until soft. If the onion will stick to the pan, add some water and cook until water will dissolve. Add a little more water if onions are not cooked yet. Make sure that onions are soft and water is dissolved. Add salt and pepper to taste.
Prepare baking pan with parchment paper.
Stuff portobello mushrooms with cooked onions and parmesan cheese. Put on baking pan.
By the portobello mushrooms on the same baking pan lay nicely sliced zucchini, season with salt.