Step 1: Heat 2 tablespoons of olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add the chopped onion, carrots, and celery. Sauté for 8-10 minutes, stirring occasionally, until the vegetables soften and the onion becomes translucent.
Step 2: Stir in the minced garlic, dried thyme, smoked paprika, and bay leaves. Cook for another minute until fragrant, being careful not to burn the garlic.
Step 3: Add the rinsed brown or green lentils, diced potatoes (or sweet potatoes), canned diced tomatoes (undrained), and 6 cups of vegetable or chicken broth to the pot. Stir everything to combine.
Step 4: Bring the soup to a boil, then reduce the heat to low, cover, and simmer for 30-40 minutes, or until the lentils are tender and the vegetables are cooked through. Stir occasionally to prevent sticking.
Step 5: Remove the bay leaves. Taste the soup and season generously with salt and freshly ground black pepper. Adjust seasonings as needed. If a thicker consistency is desired, mash some vegetables against the pot or use an immersion blender for a few pulses.
Step 6: Ladle the hot soup into bowls. Garnish with fresh parsley or cilantro, and if using, stir in a splash of apple cider vinegar or lemon juice. Serve immediately with crusty bread.