Wash the zucchini and cut them into thin slices with a peeler of the same length. Bring the broth to the boil, cook the zucchini for 2-3 minutes in it and let it drain on a sieve. Collect the broth and measure 1/8 litre.
Peel and finely chop the shallots. Wash the thyme, dab dry and chop except for a little to garnish. Heat the fat in a saucepan, sauté the shallots and thyme in it and deglaze with stock and cream.
Bring to the boil, stir in the sauce thickener and bring to the boil again briefly. Season to taste with salt, pepper and lemon juice. Cut fish fillet into 8 slices and spread with mustard. Peel, wash and grate potatoes.
Season with salt and pepper, turn the fillets in it and press the rasp well. Heat clarified butter in a pan and fry the slices for 5-6 minutes, turning them over. Add the zucchini to the sauce, heat it up and spread it on plates.
Arrange fried fish on top and serve garnished with thyme.