Cheesy Kale Breakfast Casserole

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  • Level: Easy
  • Total: 1 hr 20 min
  • Active: 20 min
  • Yield: 8 servings
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2 tablespoons unsalted butter

1 yellow onion, diced 

16 ounces crusty sourdough, cut into cubes 

1 cup shredded mozzarella (about 4 ounces)

1 cup shredded Gruyere or Swiss (about 4 ounces), plus more for topping 

6 ounces kale, chopped (chard or spinach would always work!) 

3 1/2 cups whole milk 

1/2 cup heavy cream 

2 teaspoons Dijon mustard 

1 1/2 teaspoons kosher salt 

1/4 teaspoon grated nutmeg 

8 large eggs 

4 sprigs fresh thyme, leaves only

A few shakes hot sauce

Freshly ground black pepper 


  1. If you're planning to bake this immediately, preheat the oven to 350 degrees F. If you're prepping this the day before, no need to preheat now.
  2. In a skillet (or 4-quart braiser that you can put in the oven), heat the butter over medium heat. Add the onion and cook until softened, 5 to 7 minutes.
  3. Combine the sourdough, mozzarella, Gruyere and kale with the buttery onion mixture, either in a 9-by-13-inch casserole dish or in the braiser.
  4. In a large bowl, whisk together the milk, cream, mustard, salt, nutmeg, eggs, thyme, hot sauce and a bunch of turns of pepper. Pour into the casserole dish over the bread mixture. Top with another little sprinkle of Gruyere. Cover with foil and bake immediately or refrigerate up to 24 hours.
  5. Bake, covered, for 30 minutes. Remove the foil and continue to bake until browned on top and set throughout, about another 30 minutes.

Cook’s Note

Alternatively, you can bake this in a casserole dish that's lined with parchment that comes 1 inch above the sides on 2 sides. Let cool completely in the dish, then refrigerate overnight. Use the parchment wings to lift it out and then cut into slices. Heat a layer of butter or oil in a nonstick skillet over medium-high heat and brown the slices on both sides.