Preheat oven to 475. Fill bowl with cold water.
Boil water, add onions, and boil for 30 seconds. Transfer the onions into the bowl of water to cool them, and peel by pulling off the skins, but leave the root ends on. Place in the sauté pan and top with seitan and rosemary.
Whisk together remaining ingredients except parsley and pour over onions and seitan. Bring to a boil over high heat, then place entire pan in oven. Roast, stirring occasionally, for 25 minutes. Garnish with parsley and serve.