Preheat oven to 350F. Peel onion, slice in half on the equator. Remove centres leaving a bowl in the onion halves.
Filling: In a bowl, stir together 1/2 c breadcrumbs, corn, zucchini, carrots & beets. Firmly pack into onion hollows. Arrange in a shallow baking dish & pour water around the base.
Glaze: In a separate bowl, whisk together the glaze ingredients. Drizzle half the mixture over the top of the onions. Bake 30 minutes. Drizzle with remaining glaze & bake another 30 minutes.
Just before serving, sprinkle onions with remaining breadcrumbs & broil until the crumbs are toasted & golden brown.