Cut sirloin into small strips. Sprinkle with salt & pepper.
Place flour in a large plastic bag; add meat and shake lightly to coat.
In large skillet, melt 3 Tbsp. butter. Add the meat and brown on all sides. Remove meat from skilet and set aside.
Add remaining butter to skillet and melt over medium heat.
Add mushrooms, onions, garlic and lemon juice. Saute 3-5 minutes.
Add 2 Tbsp. flour. Cook, stirring, for one minute.
Add beef stock and bring mixture to a boil. Cook until thickened, 3-5 minutes.
Stir in mustard, tarragon, and sour cream.
Add browned sirloin to sauce and heat over low heat.
Prepare noodles according to package instructions. Serve beef over cooked noodles.