In a large cast-iron skillet, combine onion, 2 tablespoons butter, salt, and pepper over medium heat. Cook, stirring frequently, until dark golden brown, about 45 minutes.
Preheat oven to 475°. In a large cast-iron skillet, melt remaining 2 tablespoons butter over medium-high heat. Sprinkle both sides of steak with steak seasoning. Add to skillet; cook until browned on both sides, 2 to 3 minutes per side. Remove from skillet; let stand for 10 minutes before slicing.
Place bread on a large rimmed baking sheet. Spread mustard on bottom half of baguettes. Top each with 2 slices fontina. Divide steak and caramelized onions among sandwiches.
Bake until cheese is melted, 5 to 7 minutes. Top with arugula, and cover with top of baguettes.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/our-favorite-fall-soup-bread-pairings/