In a large pot over medium heat, soften the onions in 3 tbsp of the butter for 4 minutes. Add the coarsely chopped cauliflower and curry powder. Cook for 1 minute, stirring with a wooden spoon. Add the broth, coconut milk and lentils. Season with salt and pepper. Bring to a boil. Simmer for 20 minutes or until the cauliflower is very tender.