Preheat oven to 400°F (200°C).
Pat tilapia fillets dry and season both sides with kosher salt and freshly ground black pepper. Place them in a small baking dish.
In a bowl, mix melted butter, garlic, red pepper flakes, lemon juice, and lemon zest.
Pour the garlic butter mixture evenly over the tilapia fillets.
Arrange lemon slices on top of and around the fish.
Bake for 15–20 minutes, or until the fish is fork-tender and flakes easily.
Garnish with chopped fresh parsley before serving.
This Baked Tilapia is best served immediately while warm and juicy.