Start by cutting your croissants into large chunks (about 1-2 inch pieces).

If your croissants are fresh, it's helpful to let them sit out for a little while so they dry out slightly.

Stale or day-old croissants will absorb the custard mixture better, but fresh will still work beautifully.

Dice your onion and roughly chop the spinach. Next, grate your cheese and set aside until we need it.

In a large skillet, cook the bacon until crispy, then transfer it to a paper towel-lined plate to drain.

In the same skillet, cook the breakfast sausage over medium heat until browned and fully cooked.

Once both meats are ready, crumble the bacon and sausage into bite-sized pieces and set aside.

In the same skillet, add a little bit of bacon grease or olive oil and sauté the diced onion until soft and translucent, about 5 minutes.

Add the spinach and cook for an additional 1-2 minutes, until wilted.

Remove from heat and set aside.

In a large mixing bowl, whisk together the eggs, heavy cream, Dijon mustard, dry sage, all-purpose seasoning, and red pepper flakes.

Whisk until well combined and smooth.

Grease a 9x13-inch baking dish and then layer the croissant pieces evenly on the bottom.

Spread the sautéed onions and spinach over the croissants, followed by the cooked bacon and sausage.

Sprinkle half of the shredded Gruyère and cheddar cheeses over the top.

Now, pour the egg mixture evenly over the casserole, making sure the croissants are well-coated.

Gently press the croissants down into the custard with a spoon to ensure everything soaks in.

Top with the remaining cheese.

Cover the casserole with aluminum foil and refrigerate it for at least 30 minutes to allow the croissants to absorb the egg mixture.

If you have time, you can refrigerate it overnight, which will make the casserole even more flavorful.

Preheat your oven to 350°F (175°C).

Once ready, bake the casserole (covered with foil) for 30 minutes.

After 30 minutes, remove the foil and bake for an additional 15-20 minutes, or until the casserole is golden brown on top and the eggs are set.

You can check for doneness by inserting a knife into the center—if it comes out clean, it’s ready.

Once baked, sprinkle freshly grated Parmesan over the top and garnish with chopped chives for a pop of color and flavor.

Let the casserole rest for a few minutes before serving.