Place lentils in a pot and cover with 2 inches of water. Bring to a boil, then reduce heat and simmer covered for 20 minutes. Drain and set aside.
In a large pot over medium heat, warm the oil. Cook onion, celery, and bell pepper until lightly browned, about 10 minutes. Add the garlic and cook 1 minute. Stir in remaining ingredients and lentils. Simmer until thickened, about 10 minutes.
Toast buns cut-side up in the oven. Assemble sandwiches, and serve immediately with pasta salad.