To prepare beets, trim, peel, and coarsely grate the beet roots. Then separate the stems from the leaves and chop separately.
In a large pot or dutch oven, over medium heat, melt the fat or oil. Saute onions and garlic with a few pinches of salt until very soft, add cumin and cook another few minutes. Add tomato paste and a few tbsp. of the stock, combine and cook for another few mintes. Add the grated beet roots and carrots and saute until soft, about 10 minutes.
Add the rest of the stock and bring to a boil over high heat. Add lentils and beet stems. Lower heat to medium-low and simmer for about 45 minutes to an hour, until lentils are cooked through.
Adjust seasoning with salt and lemon juice. Taste the soup again and if necessary, add sugar, a couple of teaspoons at a time until the soup tastes balanced with a hint of sweetness. It doesn’t take a lot of sugar, but makes a huge difference.
Add chopped beet greens and stir to wilt. Serve hot with sour cream.