Preheat oven to 350°. Chop onion and place in deep casserole dish or roasting pan.
Cut beef into 1 inch pieces, season with salt and pepper.
Heat oil in over med-high heat 1-3 minutes or until simmering. Add beef, cook without stirring 3-4 minutes or until browned on one side. Stir and cook an additional 2-3 minutes. Remove beef from skillet to casserole dish.
Add mushrooms to skillet. Cook and stir 4-6 minutes or until mushrooms begin to soften and brown. Add mushrooms to casserole dish.
Combine broth, flour, 1/4 cup sherry, tomato paste, mustard, garlic and thyme in a small bowl. Whisk until flour is incorporated. Pour broth mixture into casserole dish, mix well.
Bake covered for 1-11/2 hours or until beef in tender, stirring once halfway through
Remove dish from oven; let stand, covered, 10 minutes or until sauce is thickened. Stir in remaining 1/4 cup of sherry, sour cream and lemon juice.
Serve over egg noodles.
