Slice the beef thin on the diagonal. I strive for about 1/8". Some people prefer to cut the strips into about 1" long pieces. I don't but do what suits you.
Combine the flour, salt and pepper in a plastic zip bag or even a small brown lunch bag. Place the beef strips in the bag, seal the bag and shake the contents until the strips are well coated with the flour. The flour helps to brown the beef strips and thicken the sauce.
Heat 2 tablespoons of oil in a 12" saute pan over medium-high heat. Brown the strips in a single layer, turning once. Make sure they are nicely browned on each side. Remove the strips to a plate as you work. If you overcrowd the beef strips they'll steam instead of brown.
Once the strips are all browned (and placed on a plate), add the last tablespoon of oil to the pan. Add the chopped onion and garlic. Saute over medium heat until the onions are tender. Add the wine to the pan and turn up the heat to medium-high. As the wine simmers scrape the bottom of the pan to dislodge any browned bits. Simmer about 2 -3 minutes until the wine reduces to about 3/4 cup. Add the beef and the mushrooms to the pan, stir to combine, reduce the heat and keep the mixture to a simmer for 30 minutes. I like to keep the pan partially covered.
Heat the water for the noodles. Cook the noodles according to the package directions and drain once cooked. If you like your noodles buttered you can put the drained noodles back in the pan and add butter.
Once the Stroganoff is cooked remove it from the heat. Place the sour cream in a small bowl and add about 2 tablespoons of sauce to the bowl. Stir the mixture to combine and temper the sour cream. Add the sour cream to the Stroganoff and stir gently to combine. Don't try to add the sour cream directly to the Stroganoff while it is still on the burner or it will curdle.
Serve the Stroganoff on top of a bed of the hot noodles and sprinkle with chopped parsley.
Most of the essentials ready to go.
The beef slices are shake in a bag with the flour, salt and pepper.
The beef sauteing until nicely browned.
Dinner is served including fresh spinach on the side.