STEP 1/7
Marinate thickly sliced sirloin with salt, pepper, olive oil and sliced onions for at least 2 hours.
STEP 2/7
Prepare mushrooms by trimming them and dividing them into two pieces, and prepare onions by mince them.
STEP 3/7
Mix all the sauce ingredients except onions and mushrooms.
Dissolve about 1ts of butter in a heated pan, stir-fry onions first, and then stir-fry mushrooms. It's okay if the mushroom doesn't die completely.
STEP 4/7
Add the mixed sauce ingredients to this and boil it, and when it starts to boil, reduce it to medium-low heat and boil it down for 2 to 3 minutes. The sauce is done like this.
STEP 5/7
Heat the pan over high heat and melt the butter, then place the meat on top and bake until the surface is golden brown. If you do this, the juice comes out less while you cook it more.
STEP 6/7
Cook the surface first, cover it, reduce the heat to low, and bake it to the desired level. I baked it for another 2 minutes or so and baked it with medium rare.
STEP 7/7
Put the sauce on a thickly baked sirloin and a plate