Bring 1/4 l water to the boil, stir in the stock. Clean, wash and cut the zucchini into thin slices, e.g. with a peeler. Cook covered in the stock for 2-3 minutes. Drain, collect approx. 1/8 l stock
Peel and chop the shallots. Wash, pluck and chop the thyme. Heat the fat. Sauté shallots and thyme in it. Deglaze with stock and cream. Bring to the boil, thicken with sauce thickener. Season to taste with salt, pepper and lemon juice
Wash the fish, pat dry and cut into 8 pieces. Coat all around with mustard. Peel, wash, grate and season the potatoes. Wrap the fish with the grated potatoes, press well
Heat the oil. Fry the fish for 3-5 minutes on each side. Heat the zucchini in the sauce. Arrange everything on plates