Heat the olive oil in a cast iron skillet or deep pan over high heat. The pan is ready when the oil looks loose and a drop of moisture would hit with a strong, sharp sizzle.

Season half the beef strips with salt and pepper, lay them in the hot pan, and leave them untouched for 1 minute before turning with kitchen tongs. You want a loud sizzle right away and a golden-brown crust on the first side, not gray steamed meat.

Brown that first batch, transfer it out, then repeat with the remaining beef and set all the meat aside once colored. Cooking in batches keeps the pan hot enough to sear, while crowding would trap moisture and soften the crust.

Add the butter to the same skillet, let it melt, then cook the sliced onion for 1 to 2 minutes. The onion should start to soften and smell sweet while still keeping some shape.

Add the sliced mushrooms and cook them over medium-high heat until golden. They should lose their raw look, release moisture, then take on deeper brown edges and a richer aroma.

Stir in the almond flour, pour in half the beef stock, and keep stirring until the flour disappears smoothly before adding the remaining stock and bringing it to a simmer. The liquid should look even and glossy, with no dry pockets of almond flour or stubborn lumps.

Lower the heat, stir in the wholegrain mustard and sour cream, then simmer until the sauce thickens to the consistency of heavy cream before returning the beef to the skillet. The finished gravy should coat the spoon smoothly, and the beef only needs a brief warm-through at this stage.

Serve the stroganoff over mashed cauliflower or your favorite low-carb noodles. The sauce should sit in a creamy layer over the base instead of running thin around the plate.