Prepare the filling. Heat oil in a frying pan. Fry the onion until golden brown. Add the mushrooms. Cook over medium heat for about 3 minutes. Add dill and pour in cream. Once the liquid has reduced by half, sprinkle with garlic and turn off the heat. Combine the mushroom mixture with bread, egg and 2/3 of the cheese. Season with salt, pepper and nutmeg. Stir.
Make stuffed tomatoes. Grease a baking dish with vegetable oil. Put a small amount of mushroom filling in each tomato. Sprinkle the remaining cheese on top. Place the dish in the oven, preheated to 180 degrees, for 20 minutes.