For the zucchini casserole, cover zucchini with water in a saucepan, add salt, bring to a boil and simmer for about 5 minutes. Drain well and puree in a blender.
Preheat the oven to 170 degrees. Melt butter in a saucepan and sauté the onion in it. Mix the sauteed onion with eggs, creme fraiche and 3 tablespoons of the cheese.
Add to the zucchini puree. Mix everything well and season with salt and pepper. Grease a baking dish and spread the mixture evenly in it.
Sprinkle the breadcrumbs and the remaining cheese on top and bake the zucchini casserole in the oven for about 40 minutes.