Heat olive oil in a large saucepan over medium heat. Add diced onion and cook for 5-6 minutes until softened and translucent.
Add minced garlic and cook for 1 minute until fragrant, stirring constantly to prevent burning.
Stir in tomato paste and cook for 2-3 minutes, stirring frequently, until it darkens slightly and becomes caramelized.
Add crushed tomatoes, diced tomatoes, dried basil, oregano, sugar, salt, black pepper, and red pepper flakes. Stir well to combine.
Bring sauce to a simmer, then reduce heat to low. Let simmer uncovered for 25-30 minutes, stirring occasionally, until sauce has thickened.
Taste and adjust seasonings as needed. Stir in fresh basil just before serving over your favorite pasta.