Heat oil and butter in a large saucepan over medium heat.
Add onion and cook stirring a few times until they begin to get translucent, add the minced garlic and grated ginger, cook stirring until softened, about a couple of minutes.
Add the curry and turmeric and cook stirring until the spices are fragrant, about 1 minute.
Add the chicken (or vegetable) broth, the red lentils and the chopped sweet potato. If you are also using coconut milk, add it here.
Bring to a boil and then lower heat to a simmer and simmer the soup for approximately 15 minutes, stirring now and then so the lentils don’t stick to the bottom of saucepan.
When the sweet potato and lentils are tender, add 2 tablespoons of freshly squeezed lemon juice, salt and freshly ground pepper to taste.
You can either keep the soup chunky or using a stick blender you can purée part of the soup or all of it if you prefer a creamier texture. You can also transfer the soup in batches to a blender and purée until smooth. Pour the soup back into the saucepan and adjust the seasonings and consistency of the soup with more broth or water.
Garnish the soup with a dollop of yogourt and serve with lemon wedges (optional).
You can use regular red lentils for this soup but they take twice as long to cook as split lentils.
Make this soup vegan by using only oil to cook the aromatics and vegetable broth for the soup.
If you are adding coconut milk to this soup, reduce the amount of broth by the same amount of coconut milk you are going to use (one regular sized can contains about 40 ml). You could always add more broth to thin out the soup if you find it too thick.