Preheat oven to 350 degrees. In a large saucepan, heat 1 tablespoon oil over medium. Add onion and garlic; season with salt and pepper. Cook, stirring occasionally, 1 minute. Add carrots, lentils, broth, bay leaf, cayenne, and 3 1/2 cups water. Bring to a boil, reduce heat, and simmer, partially covered, until lentils are just tender, about 20 minutes. Discard the bay leaf.
Meanwhile, on a rimmed baking sheet, toss bread with remaining tablespoon oil; season with salt and pepper. Bake until golden brown, 5 to 7 minutes, tossing once halfway through. Serve soup topped with croutons.
A look at lentils: This earthy-tasting member of the legume family is available in a range of colors and sizes. Lentils are a good source of protein, iron, and phosphorus, and keep for up to a year in an airtight container.