The day before, cut the bread into 1-inch dice. Spread out on a baking sheet and let dry overnight. Heat a large saucepan over medium-low heat and add the olive oil, onions, 1/2 tsp. of the salt and 1/4 tsp. of the pepper. Cook the onions until translucent, 8 to 10 minutes. Stir in the vinegar and honey. Reduce the heat to low, cover and cook, stirring every 10 minutes, until all the liquid has evaporated and the onions are brown, about 1 hour. Preheat an oven to 350°F. On a rimmed baking sheet, toss the mushrooms with the vegetable oil and the 2 Tbs. melted butter. Roast, stirring occasionally, until the mushrooms are browned and tender, 25 to 30 minutes. Drain well on paper towels and let cool to room temperature. Butter an 11-by-14-inch ceramic casserole dish. In a large bowl, toss the bread with the remaining 1 1/2 tsp. salt, 1 tsp. pepper, 1 stick melted butter, stock, onions, mushrooms, sage, parsley, chives and hazelnuts. Transfer the stuffing to the prepared dish and cover tightly with aluminum foil. Bake for 20 minutes, then remove the foil and continue baking until golden brown, 35 to 45 minutes more. Serves Adapted from a recipe by Chef Trevor Kunk, Blue Hill, New York City.