Preheat the oven to 400°F. Combine the butter and brown sugar in a bowl and microwave for 30 seconds, until the butter and sugar have melted together. Stir in the mustard, honey, and soy sauce. Heat the oil in an ovenproof skillet over high heat. Season the salmon with salt and pepper and add to the pan flesh-side down. Cook for 3 to 4 minutes until fully browned and flip. Brush with half of the glaze and place the pan in the oven until the salmon is firm and flaky (but before the white fat begins to form on the surface), about 5 minutes. Remove, brush the salmon with more of the honey mustard, and serve with the asparagus.