Here’s dinner done right: Combine a bunch of simple ingredients on a sheet pan, roast them for less than 20 minutes, then sit down to an outstanding meal of tender salmon and perfectly cooked asparagus with a luscious, lemony mustard sauce.
Heat the oven to 425°F (220°C). Line a rimmed baking sheet with parchment paper or aluminum foil.
Place the asparagus and shallot on the prepared baking sheet, drizzle with 1 tablespoon of the olive oil, and toss well. Spread them out, sprinkle them with 1/2 teaspoon salt and 1/4 teaspoon pepper. Roast for 8 minutes.
Meanwhile, season the salmon with 3/4 teaspoon salt and 1/4 teaspoon pepper.
Halve the lemon. Zest and juice one of the lemon halves into a small bowl. Add the remaining 3 tablespoons of olive oil, both mustards, and the dill. Set the mustard sauce aside. Slice the remaining half lemon into thin wheels; set aside.
Remove the asparagus from the oven. Toss them on the baking sheet, then tuck the salmon, skin-side down, between the asparagus, ensuring there’s space between each fillet. Scatter the lemon wheels over the asparagus. Spoon the mustard sauce over the salmon and drizzle any remaining sauce over the asparagus. Roast for 8 to10 minutes, depending on how thick your salmon is and how well you like it cooked; use a paring knife to peek inside and check for doneness; medium-rare salmon has a more translucent-pink center, and medium-well is more opaque. Serve immediately.