1) Preheat oven to 400 degrees and line large flat baking pan with parchment paper (make sure parchment paper is not hanging over edges, cut with scissors if necessary to fit pan or it can burn)
2) In a small bowl mix together all butter ingredients (it will take a few mins for lemon juice to soak into butter, but keep stirring and it will come together). Set bowl aside
3) Place salmon filet (skin side down) onto parchment paper lined pan and place asparagus (rinsed with ends snapped) around the filet. Sprinkle salt and pepper on filet and asparagus, then drizzle asparagus with olive oil lightly
4) Use a spoon and knife to scoop and spread butter mixture over entirety of salmon filet (should appear to be a thick layer)
5) Place slices of lemon on both asparagus and filet for additional flavoring. Place pan into oven and bake for 13-15 mins depending on size of salmon filet. After thoroughly cooked, turn oven onto broil setting and broil for 2-3 minutes until golden brown