Bring a large pot of salted water to a boil over high heat. Add spaghetti and cook until al dente ( 7 to 9 minutes).
While spaghetti is cooking, melt 6 Tbsp. butter in a large high-sided skillet over medium heat. Add the garlic and cook, stirring often, until just beginning to brown; about 1 minute. Using a slotted spoon, transfer the garlic to a small bowl.
Add tomatoes to the skillet. Reduce heat to low and cook, stirring occasionally until the tomatoes are softened and begin to burst; about 20 minutes.
Add 1 Tbsp. olive oil to ensure that the sauce stays nice and saucy. Add a pinch of crushed red pepper. Stir in reserved browned garlic slices and 1/2 cup grated parmesan into the tomatoes; season with salt. Remove from the heat. Add 1 cup of chopped fresh basil. Stir well to incorporate all ingredients.
Add the reserved cooking liquid 1/4 cup at a time, as needed, to create a perfectly silky sauce. Sprinkle with more basil and a bit more parmesan cheese.