Heat olive oil in a large Dutch oven over medium-high heat until shimmering. Add onions, carrots, and celery, and cook, stirring occasionally, until softened but not browned, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Add lentils and stir to combine. Add bay leaf, water and a large pinch kosher salt. 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 .
