Heat the olive oil in a medium saucepan over medium heat. Once hot, add the chopped onion and cook until translucent, about 5 minutes.
Add the minced garlic and crushed red pepper flakes, cooking until fragrant, about 1 minute.
Stir in the whole, peeled tomatoes, salt, and black pepper. Use a spoon to break the tomatoes into smaller pieces.
Bring the sauce to a simmer and reduce the heat to low. Let it cook, uncovered, for about 25 minutes, or until the sauce thickens.
A few minutes before finishing, stir in the torn basil leaves for fresh flavor.
Taste and adjust seasoning with additional salt, pepper, or red pepper flakes if needed.