Cook the tortellini according to the instructions on the packet. Cut the mozzarella into small pieces. Finely chop the onion.
Sweat the onion in melted butter in a pan until translucent. Add the cream and heat briefly. Add the 3 types of cheese and let them melt over reduced heat. Salt and pepper.
Wash the tomatoes and cut them in half. Wash the sage and cut into thin strips. Melt some butter in a saucepan and sauté both briefly in it.
Mix the tortellini with the sauce, add the tomatoes and serve.