Crusty sourdough bread, cheese, eggs and kale are combined for this hearty casserole for breakfast or brunch.
Alternatively, you can bake this in a casserole dish that's lined with parchment that comes 1 inch above the sides on 2 sides
If you’re planning to bake this immediately, preheat the oven to 350°F. If you’re prepping this the day before, no need to preheat now.
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.
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.
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.
Bake, covered, for 30 minutes. Remove the foil and continue to bake until browned on top and set throughout, about another 30 minutes.