How to Make It
Preheat broiler to high.
Using a spoon, remove zucchini pulp, leaving a 1/4-inch-thick shell. Coarsely chop pulp, reserving 2 cups; discard remaining pulp. Place zucchini halves in a large microwave-safe bowl. Cover with plastic wrap; vent. Microwave at HIGH for 4 minutes; let stand, covered, until time to stuff.
Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add onion and sausage to pan; cook 4 minutes or until done, stirring to crumble sausage. Stir in reserved 2 cups zucchini pulp. Place zucchini halves on a jelly-roll pan; sprinkle with 1/4 teaspoon salt. Divide sausage mixture evenly among zucchini halves.
Place bread in a mini food processor; pulse 5 to 6 times or until coarse crumbs form. Add 2 ounces cheese and 1 teaspoon oil; pulse to combine. Top zucchini halves with bread mixture. Broil 1 to 2 minutes or until cheese melts and breadcrumbs begin to brown.
Combine remaining 3 ounces cheese, tomatoes, and basil in a bowl; toss to combine. Combine remaining 1/8 teaspoon salt, remaining 1 1/2 teaspoons oil, vinegar, and pepper in a small bowl, stirring with a whisk. Drizzle over tomato mixture; toss to coat. Serve with zucchini boats.