Flatten the beef with a mallet or a rolling pin, trim off the excess fat then slice into strips. Season with salt and pepper and set aside.
Place a skillet over high heat and add half of the oil. Throw in the beef (single layer) and brown on both sides, you just want to brown not fully cooked. Remove and set aside, repeat the process with the rest of the beef add the remaining oil to brown the remaining batch of the beef.
In the same pan/skillet, add butter and met on medium heat. Sauté onions and garlic for about minute, add mushrooms and cook until golden brown.
Use the ladle or cooking spoon to scrape off the bottom of the skillet, then add flour, stir and cook the flour for about 1 minute.
Slowly add the broth and stir add the same time to incorporate everything.
Add the sour cream (or Greek yogurt) and Dijon mustard and stir. Simmer on low heat allow to thicken, season with salt and pepper.
TIP: Temper the sour cream by gently stir in the sour cream in other not to let the protein curdle.
Add beef and continue to simmer for about 1 minute. Check the seasoning and readjust if necessary.
Remove from the heat and serve with your favorite, in this case, egg noodles.
Pasta, mashed potatoes goes well with this recipe.