Croissant Breakfast Casserole

Ingredients

• 2 (5-oz.) country ham steaks, trimmed, bone discarded
• 2 tablespoons olive oil, divided
• 1.5 pounds Vidalia onions or other sweet onions (2 medium onions), chopped (4 cups)
• 3 ounces baby spinach, roughly chopped (3 cups)
• 6 large eggs
• 2 cups whole milk
• 2 teaspoons Dijon mustard
• 1 teaspoon kosher salt
• 1 teaspoon black pepper
• 6 ounces fontina cheese, shredded (about 1 1⁄2 cups), divided
• 1 pound (about 15) day-old mini croissants

Instructions

1. Cook ham: Coarsely chop ham. (You should have 1 1/2 cups.) Heat 1 tablespoon of the oil in a large skillet over medium. Add ham; cook, stirring occasionally, until fat is rendered, about 4 minutes. Using a slotted spoon, transfer ham to a large bowl, reserving drippings in skillet. Add remaining 1 tablespoon oil to skillet.
2. Cook onions and spinach: Stir in onions. Cook, stirring occasionally, until deeply browned, about 30 minutes. Add spinach; cook, stirring often, until wilted, 1 to 2 minutes. Transfer onion mixture to bowl with ham; let cool 10 minutes.
3. Make egg mixture: Whisk together eggs, milk, mustard, salt, and pepper.
4. Arrange croissants, and add egg mixture: Add egg mixture and 1 cup of the cheese to bowl with onion mixture, and stir to combine. Arrange croissants, slightly overlapping, in 2 rows in a lightly greased 11- x 7-inch baking dish. Pour mixture over croissants. Sprinkle with remaining ½ cup cheese
5. Chill casserole: Cover with aluminum foil, and chill 8 hours or up to overnight.
6. Bake casserole: Preheat oven to 375°F. Uncover casserole, and place on a large rimmed baking sheet. Bake in preheated oven until golden brown and center is set, about 1 hour. Shield with foil after 25 minutes, if needed to prevent excess browning.