Rigatoni al Segreto (Rigatoni with Secret Sauce)

Ingredients (11 items)

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• 4 tablespoons olive oil
• 1 cup diced onion
• 1 teaspoon salt, plus more to taste
• 2 cloves garlic, crushed (or more to taste)
• 1 pinch red pepper flakes
• 1 (28 ounce) can San Marzano (Italian) tomatoes, blended smooth
• ½ cup water (to rinse out can of tomatoes)
• 1 (8 ounce) package uncooked rigatoni pasta
• ½ cup fresh basil leaves, thinly sliced
• 4 tablespoons cold butter, cubed
• 1 cup grated Parmigiano-Reggiano cheese, plus more for serving