1 medium spaghetti squash (about 4 pounds)
1 pound lean ground beef (90% lean)
2 cups sliced fresh mushrooms
4 garlic cloves, minced, divided
4 plum tomatoes, chopped
2 cups pasta sauce
1/2 teaspoon pepper, divided
1 tablespoon olive oil
1/4 teaspoon salt
Grated Parmesan cheese, optional
Preheat oven to 375°. Cut squash lengthwise in half; remove and discard seeds. Place squash in a 13x9-in. baking pan, cut side down; add 1/2 in. of hot water. Bake, uncovered, 40 minutes. Drain water from pan; turn squash cut side up. Bake until squash is tender, 5-10 minutes longer.
Meanwhile, in a large skillet, cook beef and mushrooms over medium heat until beef is no longer pink, 6-8 minutes, breaking up beef into crumbles; drain. Add half of the garlic; cook and stir 1 minute. Stir in tomatoes, pasta sauce and 1/4 teaspoon pepper; bring to a boil. Reduce heat; simmer, uncovered, 15-20 minutes.
When squash is cool enough to handle, use a fork to separate strands. In a large skillet, heat oil over medium heat. Add remaining garlic; cook and stir 1 minute. Stir in squash, salt and remaining pepper; heat through. Serve with meat sauce and, if desired, cheese.