Croissant Breakfast Casserole

Name: Croissant Breakfast Casserole
Ingredients: ¼ cup Challenge Unsalted 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 garlic (minced); 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)