Cook the rice in boiling salted water for 20 minutes. Peel onions and cut off a lid. Hollow out the onions and chop the inside of the onion finely. Clean and wash the mushrooms and tomatoes. Cut mushrooms into slices and tomatoes into cubes.
Drain the rice. Mix cream cheese, except two tablespoons, rice, mushrooms and tomatoes and season with salt and pepper. Fill into the onions and put the lids on top. Put them into an ovenproof dish.
Add stock and diced onion. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for one hour, cover if necessary. Wash and chop parsley. Take out onions and keep warm.
Pour the stock and milk into a pot and bring to the boil. Stir in sauce thickener and bring to the boil again. Stir in parsley and remaining cream cheese and season with salt, pepper and nutmeg.