DO-AHEAD TIP: Roast spaghetti squash.
Preheat oven to 375 degrees.
Place spaghetti squash on a lightly oiled baking sheet, cut side down; bake for 30 to 40 minutes or until tender.
Meanwhile, prepare marinara sauce: In a medium saucepan over medium heat, combine next 6 ingredients (tomato sauce through black pepper), stirring well until heated through; set aside and keep warm.
Heat the oil in a large, deep skillet over medium-high heat; add sun-dried tomatoes, bell pepper, eggplant, mushrooms and onion; sauté until almost tender; add to marinara sauce and toss well to coat.
When spaghetti squash is cooked and has cooled slightly, scrape the squash out of its shell with a fork in long, thin (spaghetti-like) strands and arrange on dinner plates; top with vegetables and marinara sauce then garnish with Mozzarella.