While your oven preheats, prepare the compound butter that will add richness and flavor to the dish. In a small bowl, combine the softened butter, minced garlic, lemon juice, fresh parsley, fresh dill, salt, and black pepper. Mix until well combined and set aside. This allows the flavors to meld while you prepare the vegetables and protein. Trim the tough ends from the asparagus (about the bottom 2 inches) and pat the salmon fillets dry with paper towels—this ensures better browning and prevents the butter from becoming watery.
Preheat your oven to 450°F and line a large sheet pan with parchment paper or foil for easy cleanup. Arrange the salmon fillets in the center of the pan and position the trimmed asparagus on both sides, creating space between everything for even heat circulation. The center placement of the salmon ensures it cooks gently without drying out, while the asparagus on the sides can develop light caramelization.
Drizzle the olive oil over the asparagus spears and toss lightly to coat. Season both the salmon and asparagus generously with salt and black pepper, tasting as you go—I find that seasoning before adding the butter helps the salt penetrate the proteins better. Spoon or spread the herb butter mixture from Step 1 evenly over the salmon fillets and asparagus, reserving a small amount for finishing if desired.
Arrange the lemon slices directly on top of each salmon fillet. This adds both visual appeal and subtle citrus flavor during cooking. Place the sheet pan in the preheated 450°F oven and roast for 10-12 minutes, until the salmon is mostly opaque and flakes easily when tested with a fork. The salmon should reach an internal temperature of about 140°F at this point—it will continue cooking slightly during broiling.
After the initial roasting, switch your oven to broil and place the sheet pan on the middle rack for 2-3 minutes. Watch carefully during this step—broilers vary in intensity. The goal is to gently finish the salmon to 145°F internal temperature and add a delicate caramelization to the butter and asparagus tips. Don't walk away; these final minutes go quickly and you want to avoid overcooking the delicate fish.