Dredge the sirloin strips in flour. In a large skillet over medium-high heat, melt 1 tablespoon butter and brown the sirloin strips, about 3 minutes on each side.
Set meat aside and add remaining butter to skillet. Sauté onions and mushrooms, about 5 minutes. Return meat to skillet until to desired temperature and add salt and pepper. Stir well.
Add sherry, thyme and beef broth and bring to a boil. Reduce heat. Cover and simmer for 1 hour.
Taste and adjust seasonings, adding more salt or pepper, if desired. Remove from heat and stir in sour cream. Serve immediately with buttered egg noodles.