Steaks should be 1-inch thick or less. If steaks are too thick place a piece of plastic wrap over the top and pound with a meat mallet.
Sprinkle ¼ tsp. each of kosher salt and pepper on both sides of steaks. Allow to sit at room temperature for 15 minutes or so.
Mince 2 cloves of garlic and 2 shallots or ½ a white onion.
Wipe any excess dirt from an 8 oz. package of cremini or button mushrooms. Cut into slices.
Heat a large cast iron skillet to medium-high. Add 1 Tbsp. of vegetable oil or avocado oil. Once the pan and oil are hot add both of the steaks to the pan. Sear for 4-8 minutes on the first side (a brown crust will form on outside of steak). Flip and sear for another 4-8 minutes on the other side. Cooking times will vary depending on the thickness of your steak. The safe internal temperature for beef is 145 degrees. This can be checked using a meat thermometer. Remove the steaks, place them on a plate, cover with foil, and allow to rest.
Reduce the heat the medium and add 1 Tbsp. of butter to the pan. Once the butter is melted add the sliced mushrooms and a pinch of salt and pepper. Saute for 1-2 minutes, stirring often.
Add the garlic and onions or shallots to the skillet and saute for another 2-3 minutes, stirring often.
Remove skillet from heat, add ¼ cup of Cognac or brandy to the pan, and return to the heat. [Be careful as alcohol may cause a flame.]
Saute for 1-2 minutes and allow the alcohol to cook down.
Add 1 Tbsp. Dijon mustard, 1 Tbsp. Worcestershire sauce, and ½ cup of beef broth. Stir to incorporate.
Add ¼ cup heavy whipping cream to the skillet. Stir and allow to cook for 5 minutes or so. The sauce should thicken a bit as it cooks.
Return the steak along with their juices to the pan and cook for another 2 minutes or so. Remove steaks from pan and cut into slices.
Serve with a generous portion of the mushroom sauce and chopped chives or parsley for garnish (optional).