Preheat oven to 400°F (205°C).
Line a rimmed baking sheet with parchment paper or foil.
Pat salmon fillets dry.
Arrange salmon skin-side down, if applicable, with space between each fillet.
Combine melted butter, garlic, lemon juice, and lemon zest.
Stir in half the parsley, paprika, salt, pepper, and optional red pepper flakes.
Spoon or brush the lemon butter over the salmon.
Arrange several thin lemon slices around the fillets.
Bake for approximately 10–14 minutes, depending on thickness.
Check the thickest portion for doneness.
For food safety, cook fin fish to an internal temperature of 145°F (63°C).
Remove from the oven.
Spoon a little lemon-butter pan liquid over the salmon.
Sprinkle with remaining parsley.
Rest for 2–3 minutes.
Serve warm with fresh lemon wedges.