In a medium saucepan, heat the butter and olive oil. Add in the onion and cook for about 8 minutes until softened. Add in the red lentils, diced beetroots , diced carrots and broth. Bring to a boil, reduce to a simmer and allow to simmer for about 35 minutes.
Remove the saucepan from the heat and allow to cool. Pour the soup into a blender to cream. Season to taste again and depending on your preference for spiciness, add a dash of hot sauce. Serve in deep plates or bowls.
Mix the yoghurt with a little cumin and lemon juice and salt to taste and add one tablespoon in the center of each soup.