Bring 6 quarts of water to a boil and add 2 tablespoons salt. Place lentils in a large pot, add the potato, bay leaf and enough water to cover and rise 2 inches above the ingredients.
Bring to a boil, reduce to a simmer and cook until the potato is tender, about 15 minutes. Heat oil over medium-high heat in a heavy-bottomed skillet.
Add the onion and cook over medium heat for 5 minutes, until it softens and begins to color. Add juice from the tomatoes to the lentil pot and the tomatoes to the skillet with the onion.
Stir the tomatoes and completely break them up with a wooden spoon. Add red pepper flakes, salt and pepper to taste. Cook for 5 minutes, until the tomatoes are softened and broken down. Add this mixture to the lentils.
Cook Mezzi Rigatoni for 4 minutes less than the package recommended directions. Drain the pasta and reserve 1 cup of the pasta cooking water.
Add pasta to the lentils, along with the garlic cloves, rosemary and thyme and cook at a simmer for 5 minutes.
Season with salt and pepper, remove the herb sprigs and garlic cloves and serve, divided evenly among 6 warmed pasta bowls.