Fill a large pot with water, set it over high heat, and bring it to a boil for the pasta.

Set a large skillet over medium heat and add 1 tablespoon of the olive oil.

Put the dried lentils into the skillet in an even layer. Cook them for 4 to 6 minutes, stirring every minute or so, until they look darker and a few edges turn crisp.

Crisp the lentils: Add half of the chopped rosemary and a small pinch of the salt to the skillet with the lentils. Cook for 30 seconds, just until the rosemary smells fragrant.

Transfer the lentil mixture to a plate and set it aside. Leave the skillet on the stove.

When the water boils, add 1 tablespoon of the salt to the pot. Add the rigatoni, stir well, and cook until it is tender but still has a little bite, following the package time.

While the pasta cooks, return the skillet to medium heat and add the remaining 1 tablespoon olive oil.

Start the sauce: Add the chopped onion to the skillet. Cook for 5 to 6 minutes, stirring often, until it softens and starts to turn pale golden.

Add the sliced garlic and the remaining rosemary to the onion. Cook for about 30 seconds, stirring, until the garlic smells good but is not browned.

Add the butter to the skillet. Let it melt and bubble for 1 to 2 minutes, stirring gently, until it smells nutty and turns lightly golden around the onion.

Scoop 1 cup of the pasta cooking water into a mug or heatproof measuring cup and set it near the stove.

Drain the rigatoni in a colander right when it is ready. Do not rinse it.