Preheat the oven to 350 degrees F. Spread the halved croissants, cut side up, on a parchment lined half sheet pan and toast in the oven until golden, 10-12 minutes, checking often so they don't burn. Let the croissants cool.

Tear the croissants into large bite-size pieces and place evenly in the bottom of a 9X13-inch baking dish.

In a 10- or 12-inch skillet set over medium heat, cook the bacon until crisp. Remove the bacon with a slotted spoon to a paper-towel lined plate. Drain all but 1 tablespoon of the bacon grease and return the skillet to medium heat.

Add the shallot (or red onion) and a pinch of salt and pepper, and cook for 3-5 minutes, until translucent.

Sprinkle the cooked bacon and onions over the torn croissants followed by the shredded cheese. At this point, you can lightly toss the ingredients together to distribute the cheese/bacon/onion throughout or leave as is (I usually don't toss).

In a medium bowl, whisk together the eggs, milk, heavy cream, salt and pepper until very well-combined. Pour the egg mixture evenly over the casserole.

Cover the baking dish and refrigerate for at least 4 hours or up to 12 hours (any longer than 12 hours and the croissants can start to break down and disintegrate).

When ready to bake, preheat the oven to 350 degrees F. Remove the cover from the casserole and place the baking dish on a foil-lined sheet pan (to catch any spillovers). Bake for 45-50 minutes until the casserole is golden, puffy, and firm to the touch. Let stand 10-15 minutes before serving.