Italian-Style Spaghetti Squash

1 pound spaghetti squash, halved lengthwise and seeded
1 tablespoon olive oil
1/2 medium red onion, thinly sliced
1/2 medium zucchini, cut into 1/2 inch dice
2 medium tomatoes, diced
1/8 teaspoon salt
1/8 teaspoon coarsely ground pepper
1/4 cup reduced-fat grated Parmesan cheese (optional)
1/2 small lemon, sliced

Place the squash halves, cut sides down, in a glass baking dish. Add 1/8 cup water and cover with plastic wrap. Microwave on high for 8-10 minutes until tender; cool slightly.
Meanwhile, in a large skillet, heat 1 tablespoon of the oil. Add the onion and cook over medium-high heat for 3 minutes until the onion is translucent. Add the zucchini and cook for 4-5 minutes until the zucchini begins to brown. Add the tomatoes, salt, and pepper. Reduce the heat; simmer gently for 10 minutes.
Using a fork, scrape the squash strands into a bowl. Toss with the remaining tablespoon of oil. Mound the squash in the center of 4 pasta bowls and spoon the vegetable mixture around the squash. Drizzle with more oil, if desired, and garnish with Parmesan cheese, if using. Add the lemon slices.