In a large Dutch oven or soup pot, heat the olive oil over medium heat. Add the diced onion, carrots, and celery. Sauté for 8-10 minutes until the vegetables have softened and the onion is translucent.
Add the minced garlic, cumin, turmeric, and thyme/oregano to the pot. Cook for 1 minute until fragrant.
Add the rinsed red lentils, broth, diced tomatoes, and bay leaf to the pot. Stir to combine.
Bring the soup to a boil, then immediately reduce the heat to low. Cover the pot and simmer for 35-45 minutes. The lentils are finished when they have mostly broken down, giving the soup a creamy, thick texture. Stir occasionally to prevent sticking.
Remove the bay leaf. If the soup is too thick, add a little water or broth to reach your desired consistency. Stir in the optional lemon juice (it brightens the flavor). Season generously with salt and pepper.
