Storage onions are low in water and high in sulfur, so they store well and are available year-round. Storage onions are more pungent and flavorful than sweet onions, and they're best if cooked.
Reprinted from “The Beekman 1802 Heirloom Vegetable Cookbook” by Brent Ridge and Josh Kilmer-Purcell. Copyright (c) 2014 by Beekman 1802, LLC. By permission of Rodale Books. Available wherever books are sold.
In a large, deep skillet, heat the oil and butter over medium heat. Add the onions and cook for 35 minutes, stirring occasionally, until golden brown and soft. Add the mushrooms and cook for 5 to 7 minutes, or until the mushrooms are tender.
Preheat the oven to 350 degrees F. Brush a 9-by-13-inch baking dish with oil.
In a large bowl, whisk together the eggs, milk, cream, salt, pepper, and sage. Add the bread, the onion mixture, and Fontina and stir to combine.
Pour the mixture into the baking dish, pressing so the bread is covered with liquid, and set aside to soak for at least 10 minutes, until the bread has absorbed most of the liquid.
Bake for 35 minutes, or until the bread is crisp and the custard is set. Serve hot or at room temperature.