4 1-inch thick rib-eye steaks
1/4 cup grill seasoning (such as Montreal Steak Seasoning)
1/2 cup grainy French mustard (such as Moutarde de Meaux)
1 cup diced red onion, divided
2 tablespoons unsalted butter
Score both sides of each steak by making diagonal slices. Season with grill seasoning. Brush each side of the steak with the mustard, then press 1/2 of the red onions into the mustard on one side of the steak. Melt the butter in a large skillet over high heat. Place the steaks onion side down in the skillet. Press the remaining onions evenly into the mustard on the top side of each steak. Cook steaks until they start to firm, and are browned on the outside and red in the center, 4 to 5 minutes per side. An instant-read thermometer inserted into the center should read 125 degrees F (52 degrees C). Remove steaks from the skillet, and allow them to rest for 5 minutes before serving.