Heat the vegetable oil in a large pot over medium-high heat. Add the lamb cubes and cook until browned on all sides, about 6 to 8 minutes.
Add the chopped onion to the pot and cook for 3 minutes until it begins to soften.
Stir in the beets, carrots, and potatoes. Cook for another 2 minutes, mixing everything together.
Pour in the lamb broth. Add the rosemary, salt, and black pepper. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 1 hour and 15 minutes until the lamb and vegetables are tender.
Remove the pot from the heat. Ladle the stew into bowls and top each serving with a generous spoonful of sour cream.