Combine olive oil (1/2 cup), garlic (5 cloves) (smashed and peeled), red pepper flakes (1/2 tsp), and a handful of fresh basil (1 handful) in a small saucepan.
Heat medium-low. Once the mixture boils, let it simmer for 1–2 minutes or until the basil is wilted. Remove pan from the heat and allow the mixture to steep.
Slice onion (1) in half through the root and peel off the skins. Leave the halved roots intact for easy removal later.
Add butter (5 tbsp) to a dutch oven over medium-high heat. Once the butter is melted and beginning to brown, place the onion halves cut-side down. Let the onions sear until golden brown.
Carefully add San Marzano tomatoes (1 28 oz can) to the pot. Use a wooden spoon or a potato masher to break up the tomatoes into smaller pieces, if necessary.
Once the tomato sauce is boiling, turn the heat to low. Season with salt to taste, cover with a lid, and simmer for 20–25 minutes.
Remove onions from tomato sauce.You can leave these in if you like
Add basil oil to the tomato sauce and blend with an immersion blender. Taste and season with additional salt, as needed. If the sauce tastes too acidic, add baking soda (1/2 tsp.) to adjust the pH.
Let the sauce simmer uncovered for 10 more minutes. Once thickened to your liking, remove it from the heat and allow it to cool slightly.