For the steak: Preheat a grill pan over medium-high heat. Sprinkle the steak on both sides with the garlic powder and salt. Place on the grill pan and cook for 5 to 7 minutes; flip and cook for an additional 3 to 5 minutes, until medium rare. Remove to a cutting board and set aside to rest. Slice the steak on the bias.
For the onion rings: Fit a heavy-bottomed pot with a deep-frying thermometer and fill with 2 to 3 inches of oil. Heat the oil to 365 degrees F. Meanwhile, prepare one bowl with buttermilk and a second with flour, salt and cayenne. In batches, place the cut onion rings into the buttermilk and then into the flour; repeat one more time. Carefully place the dipped rings into the preheated oil, being sure not to overcrowd the pan. Once golden on one side, flip to the other side and fry until golden brown all around. Remove with a slotted spoon to a paper-towel-lined plate. Season with salt.
For the mustard cheese sauce: Melt the butter in a medium saucepan. Add the flour and whisk together completely to make a roux. Whisking continuously, gradually add the cold milk until completely combined and the sauce starts to thicken into a bechamel. Add the fontina, Tabasco and Dijon and whole-grain mustards. Season to taste, then remove from the heat.
To assemble the sandwich: Smear the cheese sauce on each side of the bread, top with sliced fontina, banana peppers, steak and arugula. Serve the onion rings on the side.