To prepare the meat: First, cut it into 2-inch strips towards the grain, and then cut each strip into 1/2-inch strips against the grain. Place it on a baking sheet lined with paper towels and pat-dry as much as possible. Season the meat with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Heat 1-tablespoon oil in a 12-inch skillet over medium-high heat. Cook half of the meat, 4-5 minutes on each side. Transfer the cooked meat in a bowl. Repeat the same process for the other half. Cover it with aluminum foil and set aside.
Once the meat is cooked, in the same skillet, heat the remaining 2 tablespoons oil. Add onion, mushroom, and 1/2 teaspoon salt, and cook, stirring frequently, until the juices of the mushrooms evaporate, 8-10 minutes.
Add flour and cook, stirring constantly, for a minute.
Stir in the chicken and beef broths. Scrape the bottom of the pan; making sure that all the bits and pieces of the meat is mixed in the liquid.
Return the beef into the mixture. Put the lid on and let it cook over low heat until beef is tender for 25-30 minutes.
Stir in the noodles, cover, and cook, stirring occasionally until noodles are cooked, 10 -12 minutes.
Off the heat, add the sour cream and lemon juice. Give it a big stir.
When ready to serve, garnish it with chopped thyme and/or fresh parsley.
Serve.