Cut steak across the grain into very thin strips, about 1/8 inch thick. Set aside.
Combine 1/3 cup flour, salt and pepper in plastic mixing bag. Add the bison slices, shake to coat.
Add 2 tbsp olive oil to a large non-stick skillet. Over medium heat, brown the coated bison strips, stirring constantly. Once brown, remove slices and set aside in a warm place.
Clean skillet with kitchen towel. Place over medium-low burner; put in 1 1/2 tbsp olive oil. Add onion and cook stirring occasionally for 4 minutes, or until soft and transparent.
Add mushrooms and garlic, reduce to low heat, cook and stir for another 3 minutes.
Sprinkle and stir in 1/4 cup flour. Add stock, dry sherry, and tomato paste. Blend, increase heat and bring to a slight boil. Stir in browned bison slices and cook for another minute or two.
Remove from heat and blend in sour cream. Return to stove just long enough to heat through without boiling.
Serve over cooked egg noodles