Spread the lentils on a large baking sheet and pick them over for any bits of stone.
Stir the lentils, broth, tomatoes, onion, carrots, potatoes, bay leaves, and salt in a large saucepan or soup pot. Bring to a simmer over medium-high heat. Reduce the heat to low, set a lid askew over the pan or pot, and cook until the lentils are soft, for 45 minutes.
Discard the bay leaves. Ladle the soup into bowls. Garnish each with scallions and up to 1 teaspoon olive oil.
Tip: For extra flavor, cook the chopped onion and carrot in 1 tablespoon olive oil for 5 to 7 minutes in the saucepan or soup pot set over medium heat before adding the remainder of the ingredients indicated in step 1.
Tip: Change the recipe a little by adding up to 1 cup packed baby spinach, baby kale, baby arugula, stemmed watercress, or a mixture of any to the soup after it has simmered for 35 minutes. Continue cooking with the lid askew for 10 minutes, until the lentils are tender and the greens have wilted.
Tip: For an easier prep, use frozen chopped onion (no need to thaw).
