Heat the oven to 180°C and cook the chateaubriand in the oven for 8 to 10 minutes (medium rare).
Place the chateaubriand on a wire rack to rest with some thin foil on top.
Prepare the sauce
Sweat all vegetables until translucent, deglaze with the white wine, then pour in the fond de veau.
Add the tomato sauce and a dash of the sriracha.
Cook for 4-5 minutes, season to taste and whisk in a few knobs of cold butter (monter au beurre).
Prepare the garnish
Sauté the mushrooms in hot oil for 2-3 minutes, then place in a colander.
Sauté the chopped garlic until fragrant and add the mushrooms. Season to taste and add the chopped parsley at the end.
Plating
Arrange the Arlequin sauce on the bottom of the plate, place the mushroom and a thick slice of chateaubriand on the sauce and sprinkle some fleur de sel over the beef.
Notes
Chateaubriand is a thick piece of beef tenderloin from the centrepiece, or the thick end (head).
Monter au beurre is whisking some cold butter parcels into a hot sauce to give a smooth texture and shiny finish.