Make the chile butter: Grill the peppers over high heat for about 7 minutes. Once they are charred, put them inside a covered container and let them steam until soft, about 10 minutes. Take them out and peel the skin. If you prefer a milder butter, deseed the peppers. Finely chop the peppers until they're almost a paste, then add to the melted butter. Salt to taste.
Fire up the grill. You want the coals to glow with high, even heat. Pat the fish skin dry with paper towels, then brush it with olive oil and season the outside with salt and pepper.
Place the fish on the grill and cook for about 5 to 10 minutes, depending on thickness, then flip and repeat.
Grill the lemon halves, flesh side down, until they're charred.
Test the fish for readiness: Insert a metal cake tester into the thickest part of the fish for at least 5 seconds. Remove the cake tester and immediately touch it to your upper lip. If it's hot, the fish is ready; if not, continue grilling for a few more minutes.
Remove the fish from the grill and set it on a serving plate. Let it rest for a few minutes. Add the desired amount of chile butter and top with the sliced scallions. Place lemon halves beside the fish and squeeze the juice on, as desired.