Bring a large pot of salted water to a boil, then add the pasta and cook until al dente, about 8-10 minutes. Drain and set aside.
While the pasta cooks, drain the soaked cashews and add them to a blender along with roasted garlic, chopped tomatoes, nutritional yeast, lemon juice, a pinch of salt, and a splash of water. Blend until smooth and creamy, feeling the mixture become velvety and bright in aroma.
Heat olive oil in a large skillet over medium heat until shimmering. Add chopped onion or basil stems if you like, and sauté until fragrant, about 3-4 minutes; the kitchen fills with a savory aroma.
Pour the blended sauce into the skillet, stirring well to combine with the aromatics. Let it simmer gently for about 5 minutes, watching it thicken slightly and turn a vibrant red-orange hue.
Adjust the sauce consistency with a little more water or plant-based milk if it becomes too thick, and season with additional salt or pepper to taste. The sauce should be smooth, bright, and fragrant.
Add the cooked pasta to the skillet, tossing gently to coat all the noodles evenly with the sauce. Let it cook together for another 1-2 minutes until heated through and well combined.
Finish by stirring in freshly chopped basil and squeezing a little lemon juice over the dish for a bright finish. Serve immediately with extra basil on top for a fresh look.