Onions peel, halve and, up to a 1/2 - 1 cm wide edge, hollow out with a ball cutter. Pre-cook onion halves in boiling salted water for about 10 minutes. In the meantime, roughly dice the onion pieces.
Heat 25 g fat in a pot and fry the onion pieces in it until transparent. Deglaze with broth, bring to the boil, cover and cook for about 10 minutes. Wash the basil, dab dry and pluck the leaves, except for something to garnish.
Add basil to the onion pieces and puree. Season with salt and pepper. Let it boil down for about 5 minutes. Refine with crème fraîche and pour into a flat casserole dish. Clean and quarter the mushrooms.
Heat the remaining fat in a pan and sauté the mushrooms briefly in it, set aside. Wash and dice the tomatoes and add to the mushrooms. Season with salt and pepper. Mix vegetables and cream cheese and fill into the drained onion halves.
Add to the sauce and bake in the preheated oven (electric: 200 °C/ gas: level 3) for about 30 minutes. Serve garnished with basil. Serve with rice.