In a small saucepan, gently heat olive oil over low heat. Add sliced garlic, red pepper flakes, and fresh basil. Stir occasionally for about 5 minutes, or until the garlic is fragrant and golden. Remove from heat.

In a separate medium skillet, melt butter over medium heat. Add halved onion and sauté for about 8-10 minutes, until caramelized. Remove the onion, leaving the butter in the pan.

Pour in the San Marzano tomatoes, using a wooden spoon to break them down gently. Cook the mixture for about 5-7 minutes, until it starts to bubble and thicken.

Stir in the infused olive oil, remaining garlic, and basil. Season with salt and pepper to taste. Mix for a couple of minutes to combine the flavors.

Lower the heat and let the sauce simmer gently for about 10 minutes, stirring occasionally until it thickens.

Toss the sauce with freshly cooked spaghetti. Serve immediately, garnished with fresh basil leaves.