01 - Set oven to 400°F and line a large baking sheet with parchment paper or foil to facilitate cleanup.
02 - Place salmon fillets at the center of the baking sheet and arrange asparagus spears around them.
03 - Whisk melted butter, olive oil, lemon juice, lemon zest, minced garlic, sea salt, black pepper, and optional red pepper flakes in a small bowl.
04 - Drizzle the lemon-garlic butter sauce evenly over salmon and asparagus, gently tossing asparagus to ensure thorough coating.
05 - Bake for 12 to 15 minutes until salmon is opaque and flakes easily with a fork, and asparagus achieves tender-crisp texture.
06 - Remove from oven, garnish with chopped parsley and lemon slices, then serve immediately.