Boil the pasta water: Fill a large pot with water, set it over high heat, and salt the water so it tastes lightly seasoned.

When the water comes to a full boil, add the rigatoni to the pot and stir well so the pieces do not stick together.

Cook the rigatoni until it is tender but still has a little firmness in the center, following the package time as a guide.

Start the ragù: While the pasta cooks, set a large deep skillet over medium heat and add the olive oil.

Add the chopped onion, carrot, and celery to the skillet with a pinch of salt.

Cook the vegetables, stirring often, until they look softened and the onion turns translucent, 6 to 8 minutes.

Add the chopped garlic to the skillet and cook for 30 seconds, just until it smells fragrant.

Add the tomato paste to the skillet and stir it into the vegetables.

Cook the tomato paste, stirring, until it darkens from bright red to a deeper brick red and starts to leave light marks on the pan, about 2 minutes.

Add the drained lentils to the skillet and stir to coat them in the tomato mixture.

Scoop 1 cup of the pasta cooking water out of the pot and pour it into the skillet.

Bring the sauce to a gentle simmer and cook for 3 to 4 minutes, until it thickens slightly.