In a medium saucepan, melt butter with olive oil over medium heat. Add seasoning of choice and cook until slightly fragrant, about 1 min. Add onion and tomatoes. Use a wooden spoon to mash whole tomatoes against the side of the pan to crush slightly. Reduce heat and let sauce simmer for 1 hour, stirring occasionally and continuing to mash any large pieces of tomato. Remove onions before serving sauce.
Meanwhile, preheat oven to 375 degrees. Place whole squash in oven while oven preheats; this will soften the skin slightly, making it easier to cut half. Once oven is preheated, carefully remove squash. Halve lengthwise and remove seeds. Place the two halves on a baking sheet, cut-side down. Roast for 35 min. then remove squash from oven and let it cool slightly. Use forks to shred each half of the squash flesh into spaghetti-like strands.
Use the roasted squash skins as “bowls” to serve the shredded squash, or transfer to bowls. Top with tomato sauce, grated cheese to taste, and fresh basil sliced into long, thin strips.