Season your steak generously with salt and pepper on all sides of the steak.
In the meantime, make your béarnaise sauce. In a saucepan, add diced shallot, and chopped tarragon with sherry vinegar. Season with salt and pepper. Bring this mixture to a boil, and then at a low simmer until the liquid has absorbed. Set aside and cool.
Once your vinegar mixture has cooled, on low heat, add in your egg yolks and water. Whisk continuously for 5-7 minutes until the yolks have doubled in size and it is a creamy and frothy texture.
Tips: *Make sure your heat is on low so you don’t scramble the eggs, and every couple minutes, remove from heat and keep whisking, controlling your heat. At this point, you shouldn’t leave your stovetop, keep whisking on and off the heat. (You can also do this on a double boiler but I prefer this method).
Once that is creamy, melt your butter in a separate pot or the microwave and slowly start adding the melted butter in sections in the egg yolk mixture. Do not add the butter all at once. Keep whisking so the butter melts in the mixture and emulsified without breaking the sauce for another 5-8 minutes.
Turn off the heat, adjust for seasoning and add a splash of lemon juice to finish off. At this point, it should be the texture of a thick creamy dressing. Set aside.
In a cast iron, heat a drizzle of oil until piping hot. Cook and sear your steak on all sides until cooked to your desired doneness. When you flip the first time, add in a sprig if rosemary. Remove, slice and serve with your béarnaise sauce!