This works really well as a vegetarian main dish too.
3 cups cubed butternut squash, cut into 3/4-in. chunks
1 tablespoon extra-virgin olive oil
1 1/2 teaspoons kosher salt, divided
1 teaspoon pepper, divided
About 4 tbsp. unsalted butter, divided
3 leeks, white and light green parts only, thinly sliced and rinsed well
2 garlic cloves, minced
3/4 pound mixed wild mushrooms, sliced
1 tablespoon fresh thyme leaves
6 cups cubed rustic white bread, cut into 1-in. cubes, lightly toasted
3 cups half-and-half
4 large eggs
1 tablespoon flour
1/4 cup shredded parmesan cheese
1 cup shredded gruyère cheese
How to Make It
Preheat oven to 375°. Heap squash on a rimmed baking sheet, drizzle with oil and 1/2 tsp. each salt and pepper, and toss to coat. Bake, stirring occasionally, until tender and golden brown, about 35 minutes. Transfer to a bowl.
Melt 2 tbsp. butter in a large frying pan over medium heat. Add leeks with 1/4 tsp. salt; cook until softened. Add garlic, cook 2 minutes, and add mixture to squash.
Melt remaining 2 tbsp. butter in same pan over medium heat. Add mushrooms and 1/4 tsp. each salt and pepper. Increase heat to medium-high and cook, stirring, until mushrooms have released their liquid and are beginning to brown, about 6 minutes. Remove from heat and stir in thyme.
Add mushrooms to squash-leek mixture. Stir in bread; scoop into a buttered 9- by 13-in. baking dish.
Whisk together half-and-half, eggs, remaining 1/2 tsp. salt and 1/4 tsp. pepper, the flour, and parmesan in a medium bowl. Pour custard over bread mixture and let stand 10 minutes. Top with gruyère, then bake, uncovered, until cheese is melted and beginning to brown and custard is just set (poke with a knife to check), 30 to 35 minutes.
Make ahead: Through step 4, 1 day ahead, covered and chilled; or finish bread pudding 1 day ahead, chill, and reheat at 375° for 30 minutes.