Heat oil in a large Dutch oven and sauté the onions over medium heat until soft. Add garlic and cook until fragrant. Add curry powder, cumin and coriander. Stir well and cook for 2 minutes. Add carrots and butternut, stir well and cook for a few minutes.
Pour in the vegetable broth and water, while scraping up any browned bits on the bottom with a wooden spoon.
Bring to a boil over high heat, then reduce the heat to a gentle simmer. Cook for 20 minutes.
Remove from heat and use a hand blender to purée the soup until smooth.
Return pot to the heat and add the lentils and kale.
Simmer for 15 minutes until lentils are cooked and soup has thickened. Stir in lemon juice and season with salt and pepper.