Lentils add a delicious earthy flavor to the stew; their starchiness helps thicken the broth. Brown lentils work fine in this recipe, but we prefer French green lentils; they hold their shape better. Feel free to cook dried beans especially for this recipe, but any leftover or canned beans—rinsed and drained—will work well.
1 cup extra-virgin olive oil, divided
1 medium-size yellow onion, chopped (about 2 cups)
5 medium carrots, cut into 1/2-inch pieces (about 2 cups)
5 celery stalks, cut into 1/2-inch pieces (about 2 cups)
1 (15-ounce) can diced tomatoes, undrained
6 garlic cloves, finely chopped, divided
2 tablespoons plus 1/2 teaspoon kosher salt, divided, plus more to taste
2 teaspoons black pepper, divided, plus more to taste
1 1/2 cups dried French green lentils
2 fresh or dried bay leaves
2 (6-inch) rosemary sprigs
5 cups water, plus more as needed
1/4 cup packed chopped fresh flat-leaf parsley
1 1/2 teaspoons grated lemon zest plus 1 1/2 tablespoons fresh lemon juice, divide
6 cups cooked mixed beans (such as cannellini, kidney, or cranberry)
Heat 1/2 cup oil in a large Dutch oven over medium-high. Add onion, carrots, and celery. Cook, stirring occasionally, until tender, 15 to 20 minutes. Stir in tomatoes, 1 tablespoon garlic, 2 teaspoons salt, and 1 teaspoon pepper. Cook, stirring often, until garlic is fragrant and tomato juices begin to caramelize, 4 to 6 minutes. Stir in lentils, bay leaves, rosemary sprigs, and 5 cups water. Bring to a boil over medium-high. Reduce heat to medium-low, and simmer, uncovered, stirring occasionally, until lentils are just tender, 20 to 25 minutes.
Meanwhile, stir together parsley, lemon zest, 1/2 teaspoon salt, and remaining 1 tablespoon garlic. Let mixture stand 10 minutes. Stir in remaining 1/2 cup oil, and season with salt to taste.
Add beans, remaining 4 teaspoons salt, and remaining 1 teaspoon pepper; stir to combine. If needed, stir in additional water to thin stew. Cook over low, uncovered, stirring occasionally, until beans are heated through and lentils are tender, 10 to 15 minutes. Stir in lemon juice, and season to taste with salt and pepper. Spoon gremolata over servings of stew.
