Prepare the onion and garlic

• Cut the onion in half, slice one half and dice the other half.
• Mince the garlic cloves and set aside.

Boil the lentils

• Bring the water to a boil over high heat in a saucepan.
• Rinse the lentils, add them to the water and cook until tender.
• Keep the lentil cooking water for the sauce.

Sear the diced onion

• Heat a pan over medium heat and add the diced onion.
• Sear, stirring, until it starts to change color and the cell walls break down to release flavor.
• Pour in some lentil water and let it evaporate.

Mix lentils with the diced onion

• Add the cooked lentils to the pan and mix well.
• Once the pan is dry, remove the lentils and onion and set aside.

Brown the sliced onion and deglaze

• Add the sliced onion to the same pan and cook until slightly browned.
• Pour in lentil water and scrape the bottom of the pan to deglaze.
• Mash a small spoonful of lentils and stir them in to thicken the liquid, then reduce by half.

Cook the pasta

• Bring a second pot of water to a boil and cook the pasta al dente.
• Reserve some pasta water before draining.

Build the tomato sauce

• Pour the tomato sauce into the pan with the onions and mix well.
• Lower the heat to medium-low and simmer until the sauce starts to bubble.
• Add the garlic and cook slowly to extract its flavor.

Combine lentils and sauce

• Add the reserved lentils and onion to the sauce and mix well.
• Stir in the pasta water to emulsify the sauce and help it cling to the pasta.

Add the pasta and plate

• Add the drained pasta to the pan and mix until well coated.
• Serve in bowls, topped with chopped fresh parsley if you have it.