Start by patting the beef tenderloin dry with a paper towel. Season generously with kosher salt and freshly cracked pepper, and place it uncovered in the fridge overnight.

In a small bowl, combine softened unsalted butter with minced garlic, finely chopped fresh thyme, and a pinch of salt and pepper. Mix well until thoroughly blended.

Remove the beef from the fridge about 2 hours before cooking.

Heat vegetable oil in a large skillet over medium-high heat. Sear the beef on all sides until beautifully browned, about 2-3 minutes per side. Let it rest for 15 minutes.

Preheat your oven to 120°C (250°F). Generously slather the beef with 3/4 of the garlic-thyme butter and roast in the oven for 40-50 minutes until it reaches an internal temperature of 53°C (127°F) for medium-rare.

After removing the beef from the oven, let it rest for 10 minutes.

In the same pan used for searing, sauté the sliced white mushrooms in the remaining fat until they're softened. Pour in chicken stock and Marsala, allowing it to reduce slightly. Finish with thickened cream and reserved roasting juices.