For the polenta, boil 450 ml of water, milk and 1⁄2 TL salt in a pot. Add polenta slowly while stirring and bring to the boil. Let it swell for 6-8 minutes at low heat while stirring continuously.
Immediately spread on an oiled baking tray approx. 1 1⁄2 cm thick to form a rectangle (approx. 15 x 20 cm). Let it cool down.
Peel and finely dice the onion and garlic. Heat 1 tablespoon of oil in a saucepan. Sauté onion and garlic in it. Add the frozen spinach and about 100 ml water. Cover and let thaw at medium heat, stirring.
Wash and halve the tomatoes. Add cream cheese and half the tomatoes to the spinach, bring to the boil and simmer for 1-2 minutes. Season to taste with salt and pepper.
Wash salmon fillet, dab dry (defrost frozen fillets) and cut into 4 pieces. Season salmon with salt and pepper and place in a large casserole dish. Spread spinach mixture on top.
Mix the remaining tomatoes, 1 tablespoon of oil and a little salt and pepper. Arrange the tomatoes around the fish. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for 15-20 minutes.
Cut polenta into 4 pieces (see fig. 2). Heat butter in a large coated pan. Fry the polenta in it until golden brown on each side. Keep briefly warm. Pour the tomatoes into a pot together with the resulting roast stock.
Add cream and bring to the boil briefly. Season to taste with balsamic vinegar, salt and pepper. Serve everything.