Halve squash lengthwise; discard seeds. Place squash on a microwave-safe plate, cut side down; microwave on high until tender, about 15 minutes. Cool slightly.
Meanwhile, in a large skillet, heat oil over medium-high heat; saute sausage, yellow pepper, onion and mushrooms until onion is tender, about 5 minutes.
Separate strands of squash with a fork; add to skillet. Stir in tomatoes and seasonings; bring to a boil. Reduce heat; simmer, uncovered, until flavors are blended, about 5 minutes. If desired, top with green onions.