Slice onions into thin rings and saute in olive oil until clear.
Slice zucchini into thin round slices, do the same with the tomatoes.
Into a Pyrex dish (the size depends upon how much zucchini you have) layer as follows: (you should have 3 layers).
One layer of zucchini (do not overlap).
Thin layer of sautéed onions in olive oil.
Sprinkle with parmesan cheese.
salt and black pepper (I use quite a bit).
Keep layering until you have parmesan cheese on top (put quite a bit on the top layer, again with salt and pepper) then bake in 350 oven for about 45 minutes or until brown around the sides and bubbly and the zucchini is clear.