Preheat oven to 375 F, and adjust the top oven rack so it's 8" from the bottom of the heating element. Line a baking sheet with aluminum foil, so that the foil slightly hangs over the edges.
Place salmon filets in the center of your baking sheet, and spread the asparagus evenly on either side. Pat salmon dry with paper towels.
In a medium bowl, whisk together the melted butter, lemon juice, garlic, parsley, salt, and pepper.
Use a pastry brush to coat the tops of each filets in 3/4 of the butter combo. Drizzle the remaining butter over the asparagus and toss to coat. Top each filet with a lemon slice.
Place a sheet of foil over the salmon, and fold the edges over themselves to enclose. Bake in the preheated oven for 15-17 minutes, until the salmon is almost completely cooked through at the thickest part.
Remove salmon from the oven, and turn the broiler to high. Remove the top sheet of aluminum foil (be careful, this will be steamy!), and continue to bake salmon for about 3 minutes, until the top is golden and the salmon is cooked through. Check the seasoning on the salmon before serving, if your filets are closer to 8 ounces you may want to add an additional 1/2 teaspoon of salt. Enjoy!