Prepare the vegetables and lentils Wash and trim the carrots, measure 8 ounces, and cut them into ¼-inch dice. Dice the onion and mince the garlic. Pick over the lentils, rinse in a fine-mesh strainer, and drain. Measure the cumin and coriander so they are ready for the hot pan.

Soften the onion Heat the oil in a 2–3-quart saucepan over medium heat. Add the onion, carrots, and ⅛ teaspoon salt. Cook for about 6 minutes, stirring, until the onion is softened and translucent. The carrots will still be firm. Reduce the heat if the onion begins to brown deeply.

Add the aromatics Stir in the garlic, cumin, and coriander. Cook for about 30 seconds, stirring, just until fragrant, then move straight to adding the lentils and broth.

Simmer until both ingredients are tender Add the lentils, 2½ cups broth, and black pepper. Scrape the bottom, bring to a gentle simmer, and lower the heat. Set the lid slightly ajar and simmer for about 20–25 minutes, beginning to check after 15 minutes. Stir along the base regularly, especially as the soup thickens. The lentils should be soft without a gritty center, and a carrot piece should crush easily with a fork. If the mixture becomes too thick before the carrots are ready, add 2 tablespoons of the extra broth and continue cooking.

Choose the texture Take the pot off the heat. Leave the soup rustic, or mash some of the tender carrots and lentils against the side with a fork or potato masher. Stir in more of the remaining extra broth, 2 tablespoons at a time, if you want a looser soup. Count any broth added during simmering toward the same extra ½ cup, keeping the total adjustment within that amount.

Finish with lemon Stir in 2 teaspoons lemon juice. Taste a cooled spoonful, then adjust salt, pepper, or lemon a little at a time. Divide between two bowls and add the optional parsley. If finishing has cooled the soup more than you like, return it briefly to gentle heat before serving.