Prick squash with fork; place on microwavable paper towel in microwave oven. Microwave on High 8 minutes; turn squash over. Microwave 8 to 11 minutes longer or until tender. Let stand 10 minutes.
Meanwhile, cook and drain spaghetti as directed on package; return spaghetti to saucepan.
Cut squash lengthwise in half; remove seeds and fibers. Reserve one half for another use. From other half, remove spaghetti-like strands with 2 forks; reserve shell. Add squash and remaining ingredients to spaghetti in saucepan; toss. Return spaghetti mixture to squash shell to serve.