01 - Set the oven to 400°F and line a large baking sheet with parchment paper or foil.
02 - Place salmon fillets in the center of the baking sheet and arrange asparagus spears around them.
03 - Combine melted butter, olive oil, lemon juice, lemon zest, minced garlic, sea salt, black pepper, and crushed red pepper flakes in a small bowl; whisk until blended.
04 - Drizzle the lemon-garlic butter mixture evenly over the salmon and asparagus, gently tossing the asparagus to coat.
05 - Bake for 12 to 15 minutes until salmon is opaque and flakes easily and asparagus is tender-crisp.
06 - Remove from oven, sprinkle with chopped parsley, garnish with lemon slices, and serve immediately.