For 2 1/2 cups cooked lentils, use 1 cup dried. Bring 3 1/2 cups water and salt to a boil in a saucepan over high heat. Pour lentils into the water while stirring constantly. Reduce heat to love; cover. Simmer until lentils are tender but still hold their shape, about 30 minutes. Drain well.
You can substitute 1 1/2 (15-ounce) cans of lentils, rinsed and drained.