Trim root end of onions; cut a ½-inch slice from stem end of onions. Place 1 onion, root end down, on cutting board. Using a paring knife, make a cut across onion, stopping within ½ inch from base of onion. (Do not cut all the way through onion.) Rotate onion and repeat twice, making 6 equal wedges around onion. Gently spread sections apart; place in a 12-inch cast-iron skillet. Repeat process with remaining onions.
In a small bowl, stir together mint, garlic powder, salt, and butter. Using your hands, spread butter mixture between onion sections and all over tops of onions.
Bake until onions are soft and browned around edges, about 50 minutes. Cover with foil, and bake 5 minutes more. Garnish with mint, if desired.
Recipe by Southern Cast Iron at https://southerncastiron.com/roasted-onion-flowers/