1.
For the filling: Chop the onion and briefly sauté in olive oil. Cut the large beefsteak tomatoes into cubes and add to the onion. Dice the bell pepper, finely chop the chili and add both to the sauce. Bring briefly to a boil, season with balsamic vinegar, salt and pepper.
2.
Preheat the oven to 220° C fan.
3.
Halve or quarter the squash and remove the seeds. Spaghetti squashes do not need to be peeled; after cooking you can scoop out the flesh (short spaghetti).
4.
Divide the filling among the hollowed-out spaghetti squashes, place in a gratin dish or on a baking sheet and bake at 220° C for 30 min (squash quarters) or 45 min (squash halves). 10 minutes before the end of baking, place mozzarella slices over the filling and gratinate.