- 2 tablespoons olive oil, plus more for the baking dish
- 2 medium onions, chopped
- 1 1/2 pounds butternut squash (about half a medium squash)—peeled, seeded, and cut into 1⁄2-inch pieces
- kosher salt and black pepper
- 2 tablespoons chopped fresh sage
- 6 large eggs
- 2 cups whole milk
- 3/4 pound soft French or Italian bread, cut into 1-inch pieces (about 7 1⁄2 cups)
- 1/2 pound Gruyère, grated (2 cups)
- Heat oven to 375° F. Oil a 2 ½- to 3-quart baking dish.
- Heat the oil in a large skillet over medium-high heat. Add the onions and cook, stirring frequently, until beginning to soften, 4 to 5 minutes. Add the squash, season with ½ teaspoon salt and ¼ teaspoon pepper, and cook, tossing frequently, until just tender, 8 to 10 minutes more. Stir in the sage. Let cool for 10 minutes.
- In a large bowl, whisk together the eggs, milk, and ¼ teaspoon each salt and pepper. Add the bread, cheese, and squash mixture and toss to coat.
- Transfer to the prepared baking dish and bake until golden brown and set, 55 to 60 minutes.