Heat a large Dutch oven over medium-high heat. Add onions, carrots, and celery, and cook, stirring occasionally, until softened but not browned, about 5 minutes (you may spray on Pam). Add garlic and cook until fragrant, about 30 seconds. Add lentils and stir to combine. Add bay leaf and water (or broth). Bring to a boil, reduce to a bare simmer, cover with lid slightly cracked, and cook until lentils are completely tender and falling apart, about 1 hour total, adding water as necessary (lentils should be completely covered at all times).