Heat the oil in a saucepan, fry the garlic and onion in it for about 5 minutes until translucent, add the mushrooms and cook for 3 minutes, stirring constantly. Add tomatoes, zucchini and vegetable broth or water and simmer for 15-20 minutes, uncovered, until the vegetables are cooked through. Season to taste with salt and pepper.
Heat the butter, milk and flour in a small saucepan, stirring constantly, until the sauce boils and thickens. Remove the saucepan from the heat, stir in half of the Gouda cheese and the parsley until the cheese melts, then season with salt and pepper.
Put the tomato mixture in a large, shallow ovenproof baking dish, place half of the lasagne plates on top and distribute the pieces of fish evenly on the lasagne plates. Then pour in half of the cheese sauce, spread the rest of the lasagne plates over it, pour in the rest of the cheese sauce and sprinkle everything with the retained cheese.
Bake the prepared lasagna in an oven preheated to 190 ° C for 40 minutes until the top layer is golden brown and bubbles. Garnish the finished dish with sprigs of parsley and serve immediately.
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