Heat the olive oil in a cast iron skillet or deep pan over high heat.

Sprinkle the beef with a bit of salt and pepper and add half of it to the pan. Cook for 1 minute without turning it, then use a pair of kitchen tongs to turn the beef.

Cook until browned, remove from the skillet and repeat with the second batch, then remove the beef and set aside.

Add the butter to the same skillet and allow it to melt, then add the onions and cook for 1-2 minutes.

Add the mushrooms and cook on medium-high heat until golden.

Stir in the almond flour, then add half the beef stock and stir continuously until the flour dissolves and there are no lumps. Add the rest of the beef stock and bring to a simmer.

Lower the heat and stir in the mustard and sour cream. Simmer until the sauce has the consistency of heavy cream, then add the cooked beef and stir to combine.

Serve over mashed cauliflower or your favorite low-carb noodles.