Heat oil in skillet and cook minced onion and garlic slowly in it 10 minutes. Add zucchini cut into thin slices. Add peeled and cut-up tomato, seasoning, and herbs. Cover, and cook until the vegetable is tender. Take from stove and cool.
Beat eggs lightly, season, and mix with cooled vegetables. Pour back into skillet, cover tightly and cook over a slow fire until the edges of the frittata pull away from the pan. If the middle puffs up, prick it with a long sharp knife [... or better yet, pull away from sides once or twice with large spoon, to let the soft middle flow outward].
When it is solid, brown lightly under a slow broiler flame in a preheated oven, cut in slices like a pie, and serve at once.