Mix the toasted panko, melted butter, and Old Bay seasoning
Press the panko mixture heavily onto the top of the flounder fillets on a parchment-lined pan
Bake at 400°F (200°C) for 12–15 minutes until the crust is golden and fish is cooked
Toss the julienned green apple, cabbage, apple cider vinegar, and celery seed together
Whisk the mayonnaise, relish, lemon juice, and dill into a thick, zesty tartar crema
Smear the bottom brioche bun with the tartar crema
Add the crispy baked flounder to the bun
Pile high with the sweet apple slaw
Cap it off with the top bun
