Preheat your oven to 400°F (200°C).

In a small bowl, combine the melted unsalted butter or olive oil with the juice of one large lemon, chopped fresh parsley, thyme leaves, freshly cracked black pepper, red pepper flakes, and salt. Whisk these ingredients together until fully combined.

Arrange the firm white fish fillets in a single layer in a shallow baking pan.

Pour the prepared herb mixture evenly over the fish fillets, ensuring each piece is generously coated.

Place thin slices of lemon atop each fillet and around the pan.

Bake for 12-15 minutes, checking for doneness at around 10 minutes; the fish should flake easily with a fork.

Remove from oven and serve immediately, spooning pan juices over the fish.