04As the potatoes cook, toss the onions with a little canola oil and salt and pepper and grill over medium-high heat until charred and soft. On the same grill, heat a cast-iron pan. Add the butter and cook until it browns. Remove from the heat, add the rosemary and give the pan a shake; the butter will foam. When it stops sizzling, add the onions to the butter and cook over low heat until they are very tender and the butter has taken on a pronounced oniony flavor.05Remove the spent rosemary sprigs. Pour the butter and onion mixture over the warm cooked potatoes and serve right away.