Crush the whole tomatoes by hand in a bowl, or add them to the saucepan and crush them with a wooden spoon. Set them aside so they are ready when the onion and garlic are softened.

Heat the olive oil in a large saucepan or Dutch oven over medium heat. Add the diced onion and cook for 5–7 minutes, stirring occasionally, until soft and translucent but not browned.

Add the garlic and red-pepper flakes, if using. Cook for 30–60 seconds, stirring constantly, just until the garlic is fragrant and the flakes begin to perfume the oil.

Add the crushed tomatoes, kosher salt, and black pepper. Stir well, then bring the mixture to a gentle simmer rather than a rapid boil.

Simmer the sauce uncovered for 30–40 minutes, stirring occasionally, until slightly thickened and the tomatoes have broken down. Keep the heat moderate and adjust it as needed to prevent aggressive bubbling.

Stir most of the basil into the sauce during the final 5 minutes of cooking. Taste carefully and adjust the salt; add the optional sugar only if the tomatoes taste notably acidic.

Remove the saucepan from the heat and stir in the remaining basil. For a smooth sauce, blend carefully with an immersion blender, or leave it chunky for a rustic texture.

Toss the finished sauce with 12–16 ounces of cooked pasta. Serve with a drizzle of olive oil and grated Parmesan or Pecorino, if desired.