Heat a grill to medium-low heat. Grill the chiles until blistered and softened all over.
Remove the rind from the pineapple and slice into1-inch-thick rounds. Brush the pineapple slices with canola oil and sprinkle with a little sugar. Grill until caramelized on all sides and just soft, about 10 minutes total. Remove and let cool slightly.
Roughly chop half of the pineapple slices and 1 of the chiles. Put in a food processor with the butter and process until almost smooth. Add 2 of the green onions and the chile de arbol and pulse just to combine. Taste. If the pineapple isn't very sweet, add some honey and pulse just to combine. Transfer to a bowl, cover and refrigerate until firm and the flavors meld, at least 1 hour and up to 24 hours. Let it warm slightly before grilling the fish.
Finely dice the remaining pineapple and chile. Put them in a bowl with the remaining green onions, the olive oil, lime juice and some salt and pepper. Toss to combine. Set aside while you grill the fish.
Increase the heat of the grill to high. Brush the fish with canola oil and sprinkle with salt and pepper. Grill, flesh-side down, until charred and you can move it with a fish spatula without the flesh sticking to the grill, about 3 minutes. Flip, and smear some of the pineapple butter on the flesh side. Grill the second side until charred, about 3 minutes.
Transfer to a platter, smear on a bit more of the butter mixture, top with the pineapple relish and serve immediately.