1 pound challah, cut into 3/4-inch cubes
3 tablespoons unsalted butter, plus more for dish
1 pound assorted mushrooms (such as cremini, oyster, shiitake), cut into slices
4 medium leeks, washed, halved lengthwise, then cut into 1/4-inch slices
2 garlic cloves, minced
1 teaspoon kosher salt
1/2 teaspoon chopped dried thyme
6 large eggs
2 cups heavy cream
1 cup whole milk
3/4 cup shredded mozzarella cheese
1/4 cup grated Parmesan
Heat oven to 350 degrees F. On a baking sheet, arrange bread cubes in a single layer and bake, stirring once, until light golden brown, about 12 minutes; set aside.
Meanwhile, in a large skillet, melt butter over medium heat. Add mushrooms and cook, stirring occasionally, until golden brown, about 8 minutes. Add leeks, garlic, 3/4 tsp salt and the thyme. Cook, stirring occasionally, until softened. Transfer to a bowl; let cool. Butter a 2 quart baking dish; set aside.
In a bowl, whisk together eggs, cream, milk, cheeses, pepper and remaining 1/2 teaspoon salt.
Add bread and egg mixture to mushrooms; stir to combine. Transfer to prepared baking dish, cover with plastic wrap and refrigerate 2 hours.
Heat oven to 350 degrees F. Unwrap dish; bake until golden brown and egg mixture is set, about 50 min. remove from the oven and let stand 20 minutes before serving.