1. Preheat oven to 400 degrees F.
2. Toss squash, bell pepper, and garlic with olive oil, salt, and pepper. Spread vegetables on a baking sheet and roast until soft and slightly brown (20 minutes).
3. While the squash mixture is cooking, combine the diced tomatoes and onions. Add 1 tablespoon of cilantro. Set aside.
4. Reduce oven temperature to 350 degrees F.
5. Spread tortillas on parchment-lined baking sheets. Place 1/4 cup mozzarella on one-half of each tortilla. Distribute roasted squash mixture evenly on top of the cheese, followed by green onions and remaining cilantro. Fold tortillas in half to form quesadillas. Bake until cheese is melted (5 minutes).
6. Serve quesadillas with tomato mix on the side.