Brown the roast in oil or tallow on all sides in a large dutch oven or heavy pot over medium-high heat.
Lower the heat to medium and add diced onions and garlic and cook until soft and translucent.
Add the tomato paste and stir until mixed well with the onions and garlic.
Pour in sherry to deglaze the pan, then add the stock to the pot and bring to a boil before lowering to a simmer.
Let the roast cook for 1-2 hours, until the meat is tender and pulls away from the bone easily.
Meanwhile, preheat the oven to 400F. Peel and chop the beets, carrots and potatoes into 1/4 to 1/2 inch pieces and toss in oil and salt to coat. Spread on a baking pan and roast until tender, about 30 minutes.
Once the meat is tender and slips easily from the bone, pull it out of the pot and cut it into 1/2 inch pieces, then return to the liquid along with the roasted vegetables, sliced cabbage, and 1/2 cup of chopped dill.
Let simmer for another 15-20 minutes, until cabbage is tender. Taste and add a splash of vinegar and salt and pepper to taste. This soup is excellent now but will be even better the next day once the flavors have mingled more. Serve with a dollop of sour cream and more dill.