Melt the butter in a pan over high heat, but without browning. Add in the flour and stir it for approx. 2 minutes. Whisk the milk, adding it in little by little. Cook the sauce while stirring for approx. 5 minutes. Add the cheese, salt and pepper. Rinse spinach leaves in cold water and place them in a saucepan. Steam them over a high heat for approx. 2 minutes. Put them in a sieve to drain any water.
Melt the butter over high heat, but without browning. Sauté the onions for approx. 1 minute. Add the mushrooms, salt and pepper and fry over a high heat for approx. 6 minutes, stirring occasionally.
Clean the fish. Halve the fillets lengthwise and sprinkle with salt and pepper. Fold the fish fillets together, with the skin-sides facing inward.
Arrange spinach and mushrooms mixture in the bottom of 4 small oven dishes (approx. 20 cm in diameter). Place a fish in each over dish and pour the cheese sauce over it. Sprinkle with breadcrumbs. Set the small oven dishes on the oven tray and broil in the middle of the oven for approx. 7 minutes at 225°C. Turn on the top broiler and move the oven tray up to the top of the oven for approx. 2 minutes or until golden. Serve immediately.