Melt 1 Tbsp. butter in saute pan over medium high heat. Add beef and sear til browned, 2 to 3 minutes. Transfer beef to a plate using a slotted spoon. Melt remaining 2 Tbsp. butter in same pan over medium high heat. Add mushrooms and saute til beginning to brown, 3 minutes. Add onion and cook 3 minutes, then stir in tomato paste and garlic; cook 1 minute.
Sprinkle mushroom mixture with flour and cook 1 minute. Deglaze pan with sherry, scraping up any browned bits, stir in stock and bring to boil, stirring frequently. Stir in beef, return to a boil, reduce heat to low and simmer til slightly thickened, 5 minutes
For the sour cream, mix sour cram, horseradish, dill and lemon juice; season with Tabasco, salt and pepper. Serve stroganoff over cooked white rice with a dollop of horseradish sour cream.