• Preheat your oven to 375°F. Grease a 9-inch x 12-inch baking dish and set it aside. You can use an oval dish, which has less volume, and your casserole will be a little thicker.
• In a medium, heavy-bottom sauté pan, cook the cubed pancetta, diced bacon, or bulk sausage over medium-high heat until the fat is rendered and the meat is cooked through (about 3 minutes).
• Remove the cooked meat from the pan, leaving behind any rendered fat. If you’ve used cubed pancetta, add 2 tablespoons of olive oil to the pan. If bacon, add just 1 tablespoon of oil.
• Add the diced onion, sliced mushrooms and salt to the pan, and stir to combine.
• Cover the pan and cook until the onions are translucent and the mushrooms are tender (about 5 minutes). Remove the cooked vegetables and set them aside to cool until no longer hot to the touch.
• In a large bowl, place the milk and eggs, and whisk to combine well. Add the dry pancake mix and whisk until smooth.
• Add the hash brown potatoes and about 6 ounces of the shredded cheese, and mix to combine.
• Add the cooked and cooled pancetta or bacon, onion and mushrooms. Mix gently to combine.
• Pour the mixture into the prepared baking dish. Scatter the remaining shredded cheese evenly on top.

Make Ahead Option

• At this point, the dish can be covered tightly with plastic wrap and stored in the refrigerator overnight. Uncover and allow to sit at room temperature for about 10 minutes before baking the next morning.

Bake

• Place the baking dish in the center of the preheated oven and bake until lightly golden brown on top, and firm to the touch (about 30 minutes).
• Remove from the oven and allow to set for 5 minutes before slicing and serving warm.