Preheat oven to 200C/180C fan-forced. Cut onions in half lengthways. Remove skin. Trim tops (leave bases intact). Remove 3 or 4 inner layers from cut-side of each onion half.
Place butter, bread, garlic, walnuts, sage and sugar in a bowl. Season with salt and pepper. Stir to combine. Spoon mixture into onion halves, pressing down firmly with the back of a spoon.
Place onion halves, stuffing-side up, in a baking dish. Bake for 10 minutes or until golden. Reduce oven to 160°C/140°C fan-forced. Bake for 15 to 20 minutes or until soft (cover loosely with foil if over-browning). Serve.
Recipe courtesy of taste.com.au
Serving size 6