
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 one side of the grill to medium-high heat, with a cooler side for gentler cooking.
Rub the fish fillets with olive oil, garlic powder and salt and pepper.
For Skin On Trout: Grill the trout skin side down directly on the grill grates for 4-5 minutes (or longer, as needed) to get crispy skin, then flip the trout and grill flesh side down on the cooler side for another 4-5 minutes, or until cooked through.For Skinless Trout: Grill the trout 4-5 minutes per side, until cooked through to your preference.
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.