Finely chop the onion and fry in butter until transparent, add the zucchini, cut into thin rings, fry for another 5 minutes.
Arrange vegetables in a baking dish. Beat eggs with sour cream, add salt, pepper and finely chopped basil leaves. Pour over vegetables. Sprinkle cheese on top. Bake in a preheated oven at 180 degrees for 20-25 minutes.