How to Make It
Preheat the oven to 375°. Grease a 9-by-13-inch ceramic baking dish with butter. On two large rimmed baking sheets, toss the mushrooms with the poblanos, red onions and the olive oil. Season with salt and pepper and toss again. Roast for about 25 to 30 minutes, until lightly browned and tender. Let cool.
In a large bowl, beat the eggs with 2 1/2 cups of the chicken stock, 1 1/2 teaspoons salt and 3/4 teaspoon pepper. Add the bread and vegetables and mix well. Scrape into the prepared baking dish, cover tightly with foil and let stand at room temperature for 1 hour.
Bake the stuffing for 30 minutes, until hot. Uncover and bake for 20 to 25 minutes longer, until lightly browned on top. Let stand for 10 minutes before serving.