Rinse flounder fillets and pat dry with paper towels.

In a small shallow bowl, combine panko breadcrumbs, kosher salt, paprika, and thyme leaves.

Stir to mix the seasoning.

Dredge each fish fillet on both sides in the seasoned panko mixture, ensuring they are well coated.

Melt 2 tablespoons of unsalted butter and 2 tablespoons of olive oil in a large heavy skillet over medium-high heat.

Once the skillet is hot, carefully place 2 or 3 fillets at a time in the skillet, ensuring they are in a single layer.

Cook each side until lightly golden brown, about 2-3 minutes.

Carefully turn the fillets with a wide spatula and cook for about 2 minutes longer.

Season the cooked fillets with freshly ground black pepper and keep them warm.

Continue cooking the remaining fillets in batches.

Discard any excess fat from the skillet.

Add fresh lemon juice to the skillet, scraping the pan with a wooden spatula to deglaze and loosen any flavorful bits.

Pour the warm lemon sauce over the cooked flounder fillets.

Serve immediately.

Garnish with thin lemon slices if desired.