Add mushrooms and pinch of salt and pepper to still-hot filet pan and sauté for about 90 seconds, then add shallots and garlic and sauté for another 30 seconds.
Then turn off burner, add cognac or brandy, and deglaze pan. Chef’s Note: Alcohol will produce flames once added, use caution.
Wait for flames to die down, then set burner to medium-high again.
Cook until alcohol is reduced to 1/3 in volume, about 1 minute, then add tomato paste and cook for about 30 seconds.
Add beef stock, heavy cream, Dijon mustard, and Worcestershire sauce and whisk all ingredients until fully incorporated; bring to a boil and then reduce to medium heat. Cook for about 4 minutes, until reduced to 2/3 in volume and sauce-like consistency.
Stir in chives and season to taste with salt and pepper.
Plating: Place garlic mashed potatoes on plate first, top with seared filet, and finish with Diane sauce.