Preheat the oven to 400°F (204°C).

Zest and juice one lemon. Thinly slice the second lemon and set the slices aside.

Arrange the 4 tilapia fillets in a single layer in a baking dish. Season both sides of each fillet with salt and pepper.

Melt 4 tablespoons butter in a small saucepan over low heat or in a microwave-safe bowl in the microwave.

Stir the melted butter together with 3 minced garlic cloves, 3 tablespoons chopped parsley, 1/4 teaspoon red pepper flakes, and the lemon zest and lemon juice from step 2 until combined.

Pour the lemon-butter mixture evenly over the tilapia. Use a brush or spoon to make sure each fillet is fully coated.

Arrange the lemon slices on top of the coated fillets.

Bake the fish in the preheated oven for 10 minutes.

After 10 minutes, switch the oven to broil and broil the fish for 2 minutes to finish (watch closely to avoid burning).

The fish is done when it easily flakes with a fork. Remove from the oven and serve.