Brown beef - Place a skillet with oil over medium-high heat. Add beef and saute until beef is golden brown on the outside, 5 to 7 minutes.
Slow cook borscht - In the bowl of a slow cooker combine beef, onions, beets, carrots, potatoes, stock, tomato paste and bay leaf. Cook on low for 6 to 8 hours. (If not using a slow cooker, cook in a Dutch oven. Saute onions in heated oil for ~3 minutes and then add all remaining ingredients except cabbage and yogurt. Bring to a boil and then simmer until beef is tender, ~30 minutes.)
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