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 sauce.
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.
Finely slice fennel using a mandoline, or the grater attachment on your food processor. In a large mixing bowl combine the lemon juice, mayonnaise, and sour cream and stir to combine. Add the finely sliced fennel and coleslaw mix, herbs, and olive oil and fold together. Season with salt and pepper.