Step 1: In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion, carrots, and celery. Sauté for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent.
Step 2: Add the minced garlic, dried thyme, and cumin. Cook for another minute until fragrant, being careful not to burn the garlic.
Step 3: Stir in the rinsed brown or green lentils, diced tomatoes (undrained), and vegetable or chicken broth. Add the bay leaf. Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 30-40 minutes, or until the lentils are tender. Stir occasionally to prevent sticking.
Step 4: Once the lentils are tender, remove the bay leaf. Season generously with salt and black pepper to taste. If the soup is too thick, add a little more broth or water until it reaches your desired consistency.
Step 5: For a little extra brightness, stir in a squeeze of fresh lemon juice at the very end.