Make the bechamel cheese sauce: Melt
the butter in a saucepan
over medium heat. Add the flour and stir constantly to incorporate - you do not want any color. Gradually pour in the milk as you stir, and whisk
out any lumps. As the mixture thickens up continue to stir until it reaches a boil - this ensures the flour is cooked completely. Dump in the cheese and stir for another minute until the cheese is completely melted. Stir in the horseradish
and season with salt and pepper, then set aside to keep warm.
Make arugula mayonnaise: Put arugula in a food processor
with the lemon juice and salt and pepper. Top with mayonnaise and process until well combined. Give it a taste and adjust seasoning if required.
Prepare the steak: Take the finely sliced steak and shingle it out into 2 portions on a clean cutting board lined with parchment. Put the shingled meat between 2 pieces of plastic wrap and pound out using either a saute pan or a meat mallet. After first pounding, remove the plastic wrap, fold in the edges, replace plastic and pound the meat some more. Store wrapped in plastic in the refrigerator. Now the meat is ready when you want to cook it.
Heat a large, flat grill-pan over medium heat. Split hoagie rolls in half, drizzle
with olive oil and place onto griddle
Take the meat out of the refrigerator. Peel
off 1 side of the plastic wrap and season with salt and pepper. Put this side down in the pan and then quickly peel off the other side of the plastic wrap
and season again. Cook slowly in the pan until juicy and tender and just past pink in color. Turn the meat over and then fold with some of the cheese
To serve, smear each half of the roll with arugula mayonnaise. Top the bottom with the steak and cheese, then lay a scoop of fennel slaw over the top.