Rotate the location of the onions occasionally, being careful to keep them upright. https://www.allrecipes.com/recipe/215788/grilled-sweet-onions Roast in a 400F oven for 45 to 70 minutes or until soft. Place the onions, individually on the grill grate. Place all wrapped onions into a baking dish so that they will stay upright. To roast on a grill:.