Make your basil- and garlic-infused oil by combining olive oil, garlic, red pepper flakes, and basil in a saucepan. Simmer for 1-2 minutes.
Brown the butter and sear the onion halves in a pot with butter and neutral oil over medium-high heat for 2-3 minutes.
Add canned tomatoes to the pot, break them up, and simmer for 20-25 minutes.
Purée the basil oil mixture in a food processor and add it to the sauce for the last 10 minutes of cooking.
Remove the onion halves, toss the cooked spaghetti in the sauce, and serve with Parmesan cheese and basil.
