Preheat the oven to 200°C (approximately 400ºF).
Cook the pasta in salted water until al dente and then drain.
Halve the leeks lengthwise, rinse, shake dry and cut into thin strips. Rinse finely grate the squash. Separate the garlic cloves and chop finely.
Sauté the garlic with the squash and the leeks in some oil for about 5 minutes.
Cut a lid from the upper fifth of the bell peppers. Rinse and remove the seeds and ribs.
Mix the pasta with the prepared vegetables and the cheese. Season with salt and pepper and spoon into the peppers. Cut the mozzarella into slices and distribute on top of the pasta.
Place the bell peppers close together in a baking dish and drizzle with half the vegetable broth. Replace the pepper lids and pour the remaining broth into the dish.
Bake in the oven for about 30 minutes.