Place lentils in a large pot. Add enough water to just cover. Add celery, carrot, onion and bay, and bring to a boil. Reduce heat to a simmer and cook 20 minutes, until lentils are tender.
Bring a large pot of heavily salted water to the boil.
Heat olive oil over medium heat. Add garlic and saute until it just begins to turn golden. Add the lentils (vegetables, liquid and all) and tomatoes, salt and red pepper flake. If it seems thick, add a splash of water from the pasta pot. Bring to a boil, reduce to a simmer, and cook until reduced by about a third, 10-15 minutes.
Meanwhile, add the rigatoni to the boiling water and cook until ALMOST al dente. Drain the pasta and add to the lentils to finish cooking for the last 1-2 minutes. Finish with a drizzle of raw olive oil and garnish with chopped parsley.