Cut a few slashes in the redfish fillets, season with salt and pepper and set aside in the refrigerator.
Combine fajita seasonings in a bowl.
In a medium pot, melt the butter over medium heat.
Add the onions, peppers, and 1.5 T of fajita seasoning. Stir to combine.
Cook the onions and peppers, stirring occasionally, until browned and soft, about 20-25 minutes.
Chill the onion, pepper, and butter mixture until it is spreadable, about 30 more minutes in the refrigerator or freezer. I use the freezer if I am in a time crunch and stir it every 5 or so minutes to keep it from getting too hard.
Preheat a grill to high. I use a nonstick grill tray to avoid a mess.
Remove fillets from the fridge and season them with the remaining fajita seasoning.
Divide the onion pepper butter between fillets and spread it evenly over the meat sides.
Place the fillets scale side down over the hot fire.
Cook until the fillets are cooked through at their thickest parts, about 10-15 minutes, closing the lid of the grill for the last 5 minutes to help cook the top. Enjoy!