1. Preheat oven to 450˚F with oven rack in top third of the oven. Line a large rimmed baking sheet with parchment paper and trim paper so edges to fit the pan, otherwise, it can char in the oven, especially when broiling.
2. In a small bowl, mash together all flavored butter ingredients, mashing and stirring until lemon juice is incorporated into butter then set aside. It takes a couple minutes but it will come together.
3. Place salmon filet skin-side-dow in the center of your baking pan. Rinse asparagus and break off fibrous ends by holding the base end and breaking it wherever it snaps. Arrange asparagus around salmon, drizzle asparagus lightly with olive oil. Roll asparagus to coat lightly in oil and sprinkle both asparagus and salmon with salt and pepper.
Spoon and spread ¾ of butter mixture over salmon. Dot asparagus with remaining herb butter. Scatter with slices of lemon and bake at 450˚F for 10-12 mins (smaller 1½ lb thinner fillet will take 10 min and a larger 2 lb fillet will take 12 min), then turn the oven to Broil on High and broil 2-3 minutes to give