Begin by letting your beef fillets come to room temperature. This helps them cook evenly. Pat them dry and season generously with salt and pepper.

Heat olive oil in a large skillet over medium-high heat. Once hot, add the fillets carefully. Sear for about 4 minutes on each side for medium-rare. Adjust the time based on your preferred doneness. Once done, remove the fillets and let them rest on a warm plate.

In the same skillet, add butter, followed by shallots, and garlic (if using). Sauté until softened and fragrant. The key here is to ensure that nothing burns, so keep a close eye and stir frequently.

Toss in the sliced mushrooms and thyme. Cook until the mushrooms are deeply golden and have released their moisture. This could take around 5 minutes, but it’s worth the wait!

Stir in the Worcestershire sauce and Dijon mustard. Then, pour in the cognac or white wine. Allow it to simmer for a few minutes, reducing slightly. This is when the magic happens, and you’ll notice the aroma wafting through your kitchen!

Reduce the heat and stir in the cream, mixing well. Let the sauce simmer gently until it thickens, which should take just a few minutes.

Return the steak to the skillet to coat with that divine sauce. Plate it up and don’t forget to spoon extra sauce over each fillet. Voilà, your Steak Diane is ready to shine!