Heat plenty of salted water in a large saucepan. Wash the zucchini, cut off the stem and flower base and cut in half lengthways. Hollow out to about 1 cm of the wall. Blanch for about 5 minutes in boiling salted water and drain. Peel and finely chop the onions. Cut the chicken breast fillets into fine strips.
Heat the oil in a pan and fry the chicken breast fillets briefly and vigorously, add the onions and fry briefly. Salt, pepper and set aside.
Preheat the oven to 200 °.
Roast the inside of the zucchini in the remaining frying fat, add the vegetable stock and simmer for approx. 10 minutes until soft. Stir the crème fraîche into the vegetables and puree everything finely. Season to taste with salt and pepper.
Grease a casserole dish, add the zucchini, fill with the chicken and pour the zucchini sauce on top and pour over it. Baked for about 10 minutes.