Rigatoni al Segreto (Rigatoni with Secret Sauce)

Ingredients (11)

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