Have ready a bowl of ice and cold water. Cook onions in boiling water 3 minutes and drain in a colander. Transfer onions to ice water to stop cooking. When onions are cool enough to handle, peel and reserve. Preheat oven to 425°F.
Peel remaining vegetables and cut enough into 1/2-inch cubes to measure 3/4 cup each. In a shallow roasting pan toss vegetables (except onions) with oil and roast in middle of oven until tender and golden, about 20 minutes.
In a large skillet heat butter over moderate heat until foam subsides and cook onions, stirring, until tender and pale golden. Add roasted vegetables, stock, and salt and pepper to taste and simmer until stock is slightly thickened and coats vegetables, about 2 minutes.
