Wrap squash halves in plastic wrap. Cook 1 at a time in microwave on high until tender, about 8 minutes. Pierce plastic to allow steam to escape. Cool. Meanwhile, Saute sausage, bell pepper, onion and garlic in heavy large skillet over medium-high heat until sausage browns and vegetables are tender, breaking up sausage with back of spoon, about 12 minutes. Mix in marinara sauce. Using fork, pull out squash strands from shells, leaving shells intact. Mix squash strands into sausage mixture. Season filling to taste with salt and pepper. Spoon filling into squash shells. (Can be prepared 1 day ahead. Cover; refrigerate.) Preheat oven to 400 degrees. Arrange filled squash halves on baking sheet. Sprinkle each with 1/4 cup Parmesan. Bake uncovered until heated through, about 20 minutes (30 minutes if previously chilled.) Cut each squash half in two and serve.