Using a small, sharp knife, carefully pierce the skin of the spaghetti squash every few inches. Place whole squash on a microwave safe dish and microwave for 5 minutes. Rotate and cook for 5 more minutes. Repeat once or twice more, or until the skin is easily pierced with a fork.
Meanwhile, make the pesto: add avocado, basil, lemon juice, garlic, parmesan, walnuts, salt, and olive oil to a food processor. Pulse until a smooth paste forms.
Saute shrimp and cut tomatoes on medium-high heat, or until heated through. Season with salt and pepper to taste.
Once fully cooked, use a large knife to cut the spaghetti squash in half, from stem to bottom. Carefully scrape out seeds and discard. Using a spoon, scrape the "noodles" out of the skin and place in a bowl. Add the pesto sauce and mix until spaghetti squash noodles are well coated.
Serve Shrimp and tomatoes over pesto spaghetti squash. Garnish with extra parmesan if desired.