Line a baking sheet with parchment paper or a silicone baking mat. Set aside.
Fill a pan with an inch of water. (I used my 12-inch cast iron skillet). Place over high heat. Add the pepper, bay leaf and 2 slices of lemon to the water. Bring to a low simmer. Place the tilapia in the simmering water and cook until the flesh has whitened and cooked through, about 6-8 minutes. Using a slotted spoon carefully remove the fish from water and set aside to let cool.
Empty the water from the pan. Over medium heat add the butter to the pan. Once melted add the celery, and onion. Saute the vegetables until onions turn translucent about 5-6 minutes. Add the garlic and cook 1 minute more. Transfer to a large bowl.
In a small bowl whisk 1 Tablespoon mayo, mustard, eggs, salt, pepper, paprika, red pepper flakes. Add this mixture to the sautéed vegetables.
Add the panko crumbs and chopped parsley to the large bowl and stir everything to combine.
Flake the cooked tilapia into the large bowl full of the vegetables and sauce. Form into 6 patties.
Place the patties on the prepared baking sheet, cover with plastic wrap and place in the fridge for at least one hour to set. (You can keep it in the fridge for longer, but chill for at least 1 hour).
Preheat the oven to 400°F.
Remove from fridge. Remove the plastic wrap and lightly brush the cakes with a smear of mayo.
Bake for 12-15 minutes.
Remove from the oven and serve immediately with remaining lemon wedges.