Fill a pot with water, bring to boil, and cook the pasta according to the instruction on the package.
While waiting for the pasta to be ready, prepare the sauce by putting two tablespoons of olive oil in a pan, add the garlic and stir until it become brown.
Remove the garlic, add the diced celery, carrots, and the diced tomato, salt to taste, and cover the pan.
Cook for about 20 minutes, adding water if necessary. Halfway through cooking, add the pre-boiled red lentils and the rosemary, stir and cook for remaining time.
When the pasta is ready, drain and mix with the ragu sauce.