This savory Croissant Breakfast Casserole is full of ham, mushrooms, leeks, and gruyere – it’s the breakfast casserole version of the croque monsieur!


  • ¼ cup Unsalted Challenge Dairy Butter, plus extra for buttering the pan
  • 8 ounces sliced fresh mushrooms, such as crimini or shiitake
  • 1 large leek, sliced
  • 1 medium onion, diced
  • 1 teaspoon fine sea salt, divided
  • 3-4 cloves minced garlic
  • 2 Tablespoons whole-grain mustard
  • 1 Tablespoon sherry vinegar (optional)
  • 8 large eggs
  • 2 cups whole milk
  • 6-8 ounces diced smoked ham
  • 6 ounces grated Gruyere cheese
  • 2 ounces freshly grated parmesan cheese
  • ½ teaspoon black pepper
  • 12 ounces croissants, cut or torn into 1-inch pieces


  • 1

    In a medium-sized pan, melt the butter over medium heat. Add the mushrooms, leeks, and onion with ¼ teaspoon of the salt. Saute for about 10 minutes. When the mushrooms and onions are soft and caramelized, add the garlic, mustard, and sherry vinegar and cook for 1 additional minute. Set aside to cool.

  • 2

    In a large bowl, whisk the eggs and milk until thoroughly combined. Add the cooled mushroom mixture, ham, cheeses, black pepper, and remaining ¾ teaspoon of salt. Gently fold in the croissant pieces.

  • 3

    Generously butter a 9×13-inch baking dish. Add the casserole mixture to the pan. Cover and refrigerate overnight.

  • 4

    In the morning, preheat the oven to 375ºF. Bake the casserole for 1 hour, or until the top is golden, the edges are crispy, and the center is set.