Preheat the oven to 400 degrees, slice squash in half and remove seed. Brush with a bit of olive oil, salt, and pepper. Roast on a pan, cut side down, for 45 minutes or until strings are easily pulled. Spaghetti squash may release some water while cooking, so squeeze out the squash in a clean cloth before serving.
While spaghetti squash is cooking, start your sauce. In a saucepan over medium high, heat olive oil until shimmering. Add onions and sauté until translucent. Add carrots and zucchini and continue to sauté for 2-3 minutes. Add mushrooms and continue to sauté until mushrooms have released their water. Lastly, add garlic and sauté for one more minute.
Add the tomatoes, tomato paste, salt, pepper and basil and bring to a boil. Once boiling reduce to a simmer and cook on low with lid for 30 minutes. Once sauce has reduced, add salt and pepper to taste if desired.
Top spaghetti squash with sauce and sprinkle with extra basil and parmesan cheese if desired.