How to Make It
Fill a bowl with ice water; set aside. Bring a pot of water to a boil, and add 2 teaspoons of the salt. Add squash to boiling water, and cook until just slightly softened, about 5 minutes. Drain squash, and plunge in ice water to cool. Remove from ice water, and pat dry with paper towels; sprinkle with remaining 1 teaspoon salt.
Scrape seeds from center of each squash half to create a well down the center. Arrange squash halves, cut side up on a medium rimmed baking sheet.
Preheat oven to 400°F. Heat oil in a large skillet over medium-high. Add sausage, and cook, stirring to crumble, until browned, about 5 minutes. Add mushrooms, onion, and bell pepper; cook, stirring often, until onions are translucent and mushrooms are softened, about 4 minutes. Add tomatoes, and stir until heated through, about 1 minute. Remove from heat.
Stir in mozzarella, olives, basil, and 2 tablespoons of the breadcrumbs. Divide the filling among squash halves. They will be full and mounded. Stir together Parmesan and remaining 2 tablespoons breadcrumbs in a small bowl, and sprinkle evenly over stuffed squash halves.
Bake squash in preheated oven until tender and topping is golden brown, about 20 minutes. Garnish with basil.