Buttery Croissant Strata with Spinach and Prosciutto

3 tsp. salted butter, thinly sliced, plus more for greasing

6 croissants, roughly torn into thirds

8 large eggs

3 C. whole milk

1 tsp. Dijon mustard

1 tsp. chopped fresh sage

¼ teaspoon freshly grated nutmeg

Kosher salt and freshly ground pepper

12 ounces frozen spinach, thawed and squeezed dry

1 ½ C. shredded Gouda cheese

1 ½ C. shredded Gruyère cheese 3 ounces thinly sliced prosciutto, torn

 

Preheat the oven to 350°F. Grease a 9— 13-inch baking dish. Arrange the croissants in the bottom of the baking dish and cover with the sliced butter. Bake until lightly toasted, 5 to 8 minutes. Remove and let cool in the pan until no longer hot to the touch, about 10 minutes. In a medium bowl, whisk together the eggs, milk, mustard, sage, nutmeg, and a pinch each of salt and pepper. Stir in the spinach and ¾ C. of each cheese. Carefully pour the mixture over the toasted croissants, distributing it evenly. Top with the remaining cheese and add the prosciutto to finish. Cover and refrigerate for at least 30 minutes or overnight. When ready to bake, remove the strata from the fridge and preheat the oven to 350°F.  Bake until the center of the strata is set, about 45 minutes. If the croissants begin to brown before the strata is finished cooking, cover with foil and continue baking. Remove the strata from the oven and let cool for 5 minutes before serving.