Easy Beef Stroganoff

Thinly slice the beef against the grain. Season with salt and black pepper.
Heat oil in a large skillet over medium-high heat. Add the beef in a single layer and sear until browned on both sides, working in batches if needed.
Check the skillet after searing the beef. Leave any light golden bits, but carefully wipe away dark or burnt residue. Reduce the heat to medium, add the butter, and let it melt.
Add the chopped onion and cook for about 4 minutes, until softened and translucent. Add the mushrooms and continue cooking until they release their moisture and become deeply golden.
Sprinkle the flour evenly over the mushrooms and stir for about 1 minute, until no dry flour remains.
Slowly pour in the beef stock while stirring, then add the Dijon mustard. Bring the sauce to a gentle simmer and cook until it begins to thicken.
Reduce the heat to low. Stir in the heavy cream, sour cream, and soy sauce until smooth. Season with salt and black pepper, then cook gently for about 3 minutes without letting the sauce boil.
Add the seared beef and any juices from the bowl back to the skillet. Cook gently for about 3 to 5 minutes, just until the beef is warmed through and coated in the creamy mushroom sauce.
Remove the skillet from the heat, cover it, and let the stroganoff rest briefly. Stir in the fresh parsley, taste the sauce, and adjust the seasoning before serving.