To a sheet pan or large casserole dish, add the bell peppers, tomatoes, garlic head, onion, tomato paste and cashews. Drizzle with olive oil and add salt and pepper. Toss.
Roast for 35 to 40 minutes, until the garlic, peppers and tomatoes have softened and the cashews are browned.
In the meantime, cook your spaghetti according to package directions, reserving at least 1 1/2 cups of the pasta water for later.
Once safe to handle, transfer the pepper mixture to a high speed blender with 1 cup of the reserved pasta water. Blend until smooth. I like to blend this a lot, for at least 3 to 5 minutes, to achieve a really silky sauce.
Pour the sauce over the spaghetti and toss, adding the additional 1/2 cup of reserved pasta water as needed to thin. Serve with fresh basil, vegan parmesan, more salt and pepper as needed.