Grate the mushrooms with a wet crockery stick. Finely dice the onion, slice the mushrooms and sprinkle with the juice of one lemon. Sauté the onions and mushrooms in melted butter for 5 minutes, season with nutmeg, pepper, salt and cayenne pepper. Add the parsley and chives to the mixture and pour on the whipped cream. Mix everything well and form into a gratin dish. Preheat the oven to 225 degrees. Rinse the tomatoes, cut them in half and place the halves on top of the mushrooms. Cut the feta cheese into slices, place over the tomatoes and place the gratin dish on the middle rack in the oven. Bake for 20 minutes. Serve the tomatoes garnished with basil sprigs.
Our tip: Fresh chives are much more aromatic than dried ones!