You can cook raw green or brown lentils on a stovetop or slow cooker. As a general rule, you need 3 cups of water or vegetable stock per cup of dry lentils. Place the raw lentils into a large saucepan, and cover with cold liquid. Bring to a boil, cover tightly, and reduce heat. Simmer until the lentils are tender - depending on lentil varieties, it can take between 15 to 20 minutes. Red split lentils cook way faster - more around 8-10 minutes.