Slice the onion into very thin rings. Submerge sliced onion rings in buttermilk and set aside for about 15 minutes. Mix four, salt and pepper in a large shallow bowl. Heat up oil in the large pot to 350 (or use a fryer). Take onions out of the buttermilk and shake them off, lightly. Toss onions in the flour mixture until completely coated. Fry onion straws until golden brown. Take them out onto a paper towel to catch the excess oil.
Horseradish sauce: Combine all ingredients for the sauce in a small measuring bowl and mix until completely incorporated.
Season steaks with salt and pepper on both sides. (You can use two 6-7 oz steaks or one large steak.)
Preheat a cooking pan over medium heat and add a little bit of oil for cooking.
Cook steaks until desired temperature is reached, take them out onto a cutting board and let them rest for a few minutes.
Slice steaks across the grain.
Sandwiches:
Spread some horseradish sauce on the bottom half of each baguette.
Divide steak slices among both sandwiches.
Top steak off with blue cheese crumbles, then arugula, and then onion rings.