Homemade Marinara Sauce

3 tablespoons extra-virgin olive oil

1 medium onion, chopped

4 garlic cloves, thinly sliced

2 cans (28 ounces each) peeled whole tomatoes, pureed in a food processor

1/2 teaspoon crushed red-pepper flakes

Coarse salt and freshly ground pepper

1/4 cup fresh basil, leaves torn

1 tablespoon chopped fresh oregano