In a small saucepan, combine the olive oil, minced garlic, hot pepper flakes, and fresh basil leaves. Heat over medium-low heat, allowing to cook for about 45 seconds to 2 minutes until the basil wilts. Remove from heat and set aside.

In a Dutch oven, melt the butter over medium heat. Add the onion cut side down and cook for 4 to 5 minutes until it starts to turn golden brown.

Add the San Marzano tomatoes to the pot, breaking them up into chunky pieces with a potato masher to allow their juices to mingle with the butter and onion.

Bring the tomato mixture to a simmer. Reduce the heat to medium-low, cover lightly, and let simmer for 20 to 25 minutes.

Boil water in a pot and cook the spaghetti according to the package directions, but remove it 2 minutes early.

Pour the cooled-off oil mixture into a food processor and chop to blend. Add this oil to the tomato sauce and simmer for an additional 5 minutes.

Discard the onion from the sauce. Taste the sauce and adjust seasoning with salt and pepper.

Add the drained spaghetti to the pot and cook for another 2 minutes until perfectly al dente. Transfer to a serving platter and sprinkle with parmesan cheese, fresh basil leaves, and a drizzle of olive oil.