mushroom, caramelized onion + cheddar bake

A great make-ahead dish for entertaining. Sweet onions, earthy mushrooms, and tangy mustard all work perfectly together blanketed under a gooey layer of cheese—but just about any combination of vegetables, cooked meats, and melty cheese work.

3 tablespoons unsalted butter, divided
1 large onion, thinly sliced
Kosher salt, to taste
Freshly ground black pepper
1 pound mushrooms (portobello, shiitake, button), diced  
2 tablespoons chopped fresh thyme
2 tablespoons chopped fresh flat-leaf Italian parsley
8 extra large eggs
3 1/2 cups whole milk
2 tablespoons Dijon mustard
1 loaf challah or sourdough bread, cut into 1-inch cubes
8 ounces shredded cheddar cheese

  1. Coat bottom and sides of a 13 x 9-inch baking pan with 1 tablespoon butter. Set aside.
  2. In a large sauté pan, heat 1 tablespoon butter over medium heat. Add onion and cook for 15-20 minutes, until onions are lightly browned and caramelized. Season with salt and pepper. Remove onions from pan and add remaining 1 tablespoon butter to same pan. Add mushrooms and cook for about 10 minutes, until mushrooms are softened. Stir in thyme and parsley and remove from heat. 
  3. In a large bowl, whisk together eggs, milk, and mustard. Season with salt and pepper. 
  4. Arrange bread cubes on bottom of prepared baking pan, followed by mushrooms, onions, and cheese. Pour egg mixture evenly over top. Gently press down to make sure all of bread is moistened. Cover with plastic wrap and refrigerate overnight.
  5. Remove casserole from refrigerator. Preheat oven to 350°F.
  6. Bake casserole, uncovered, for 45 minutes to 1 hour, or until golden brown on edges. (Start checking after 45 minutes and cover with aluminum foil if top is getting too dark.) Allow to cool slightly before serving. 

Serves 8-10.