1. Preheat oven to 400°F.
2. Cut squash lengthwise. Scoop out middle, rough chop and reserve it for the filling.
3. Place the squash on a rimmed baking sheet lined with parchment paper and bake for 20 minutes.
4. Meanwhile, in a large saute pan over medium-high heat, cook the onion, mushrooms, and reserved squash meat with 1 tablespoon olive oil. Cook for 5 minutes or until golden brown. Remove from heat.
5. In a bowl, combine the tomatoes, mozzarella, breadcrumbs, minced garlic, basil, Tuscan seasoning and the remaining 2 tablespoons olive oil. Stir in the onion mixture.
6. Remove the squash from oven. Fill the squash with the filling. Top with a light sprinkling of salt. Cover with the grated cheese. Bake for 10 to 15 minutes until cheese is melted and golden brown.