Bring milk, almost 3/4 l water and 1/2 teaspoon salt to the boil. Stir in corn semolina and let it swell for about 15 minutes at low heat, covered with a lid
Spread the polenta mass about 2 cm thick on a lightly greased plate. Let it cool down
Steam the beans in a little boiling salted water, covered, for 10-12 minutes. Peel and finely chop the onion. Clean the mushrooms, wash if necessary and cut into slices
Fry the onion in 1 tbsp. hot oil. Add mushrooms and fry for about 5 minutes. Drain the beans and fold into the mushrooms. Season with salt and pepper. Wash the tomatoes, cut them into quarters and remove seeds if necessary. Dice finely and fold into the vegetables
Cut polenta into 12 pieces (e.g. triangles). Heat 2 tablespoons of oil in a coated frying pan. Fry the polenta in it from both sides until golden brown. Serve with vegetables and sour cream