Preheat oven to 350°.
Cut a 3 lb. squash in half lengthwise and scrape out the seeds. Place the halves, cut side down in a large baking dish. Add 1/2 cup water to the dish and bake for 45-50 minutes, until tender.
Once cooked, let cool cut side up for at least ten minutes. Scrape the inside of the squash with a fork and presto - spaghetti strands are created! You should end up with at least 3 cups. Set aside.
Heat 2 teaspoons of the oil in a medium skillet over medium-high heat.
Season the shrimp with salt and pepper to taste. Add shrimp to pan and cook until done (4-5 minutes). Remove from pan and keep warm.
Add remaining 2 teaspoons of oil to pan and over medium-high heat, sauté onion and garlic until onion is tender.
Add squash, tomatoes, lemon juice, rosemary and dash salt. Cook for a few minutes until warmed through.
Place mixture on a plate and top with cooked shrimp. Enjoy!