Chop the onion and zucchini finely, or grate the zucchini on a grater. Put them to fry in olive oil until soft. Separate 3 egg whites from the yolks and beat them to snow with a little salt.
Mix the other 3 whole eggs and 3 egg yolks with the butter and cream. Add spices to taste and mix with the fried zucchini and onions. Finally, carefully mix with the egg white froth. Preheat a greased Teflon pan with a wide bottom and pour in the mixture.
Cover with a lid and cook on medium heat until the eggs firm up. Use the lid to flip the tortilla and then bake it finished under the lid for a few minutes on the other side.