Heat oil in a large frying pan. Stir in onions, red and green bell peppers and shallots. Cook 3 minutes. Sir in zucchini, squash, tomato, olives, and balsamic vinegar. Saute until vegetables are just tender. Season with salt and pepper to taste.
Spread 1/2 of the vegetables into an oiled 9x11 inch baking pan. Cover the vegetables with 1/2 cup of the herbs.
Arrange the filets on top of the vegetables and herbs. Spread the second 1/2 of the vegetables over the fish and sprinkle the rest of the herbs onto the vegetables. Cover the dish with foil and bake for 25 minutes.