Add the pancetta to a medium sauté pan and heat over medium heat, stirring occasionally until crispy and fat has rendered in the pan, about 8 minutes. Remove the pancetta from the pan to a large mixing bowl, reserving as much fat in the pan as possible.
Return the pan to heat, add the butter and allow to melt then add the garlic and mushrooms, sautéing until golden brown, about 6 minutes. Season with salt and pepper and add to the bowl with the pancetta. Toss together with the kale and bread.
Preheat oven to 350ºF (180ºC). In a medium mixing bowl whisk together the eggs, milk, heavy cream, nutmeg and cayenne then pour over the bread mixture, using hands to completely coat everything in the bowl.
Pour the mixture into a 9" x 12" (23cm x 30cm) greased baking dish or 10-inch (25cm) cast iron skillet and top with the cheese.
Cover the dish with foil and bake for 30 minutes. Uncover and continue baking until the cheese is golden brown and the mixture is puffed in the middle and set, about an additional 20 minutes.