Fiery Penne Pasta in Scarlatti Sauce

Ingredients

• 1 small red onion, finely diced
• 3 tablespoons olive oil
• ¾ teaspoon crushed red pepper flakes
• One 28-ounce can diced tomatoes (preferably fire-roasted)
• ¼ cup reserved pasta water
• 1 pound dry penne pasta
• ½ cup basil leaves, chopped, plus extra for garnish
• ½ teaspoon kosher salt
• 3 cloves garlic, minced