For the compound butter, soften the butter so it is malleable but not melted. Add it to a mixing bowl.
Add rosemary, lime juice, lime zest and chili flakes. Mash to combine.
Set butter onto a large piece of plastic wrap and form it into a log. Roll into the plastic and refrigerate to harden.
Heat a grill to medium-high heat.
Rub the fish fillets with olive oil, garlic powder and salt and pepper.
Grill skin sides down about ten minutes, so the skins get nice and charred. You may need a bit more time, depending on the thickness of your fillets.
Flip and grill another few minutes to ensure the trout is cooked through. You can check with a fork in a thicker section of the fish.
Remove from heat and set onto serving plates.
Top with slices of your compound butter and serve with extra lime slices and chopped parsley. The butter will melt over the hot grilled trout, making it moist and flavorful.