Preheat your oven to 350°F and grease a 9x13 inch baking dish (or similar size) with butter or cooking spray. While the oven heats, tear the croissants into 2-inch pieces and set aside. Shred the Gruyere and cheddar cheeses together in a bowl, then finely slice the chives into tiny rings and set both aside. This prep work ensures everything is ready when you need it and prevents any scrambling during assembly.

Cut the bacon into bite-sized pieces and cook in a large skillet over medium-high heat for 4-5 minutes, stirring occasionally, until the bacon is crispy and the fat has rendered. Transfer the cooked bacon to a paper towel-lined plate to drain briefly. I like using high-quality bacon like Applegate because it adds a deeper, smokier flavor that really elevates the casserole's savory profile.

Spread the cooked bacon from Step 2 evenly across the bottom of your prepared baking dish, then sprinkle half of the shredded cheese mixture from Step 1 over the bacon. Distribute the torn croissant pieces evenly over this layer, pressing down gently so they're snug but not compressed. This layering creates pockets that will soak up the custard and distribute flavor throughout the dish.

In a large bowl, whisk together the eggs, milk, cream, sage, mustard, salt, pepper, and nutmeg until well combined and smooth. Pour this custard mixture evenly over the croissant layer, making sure to distribute it throughout so every piece gets soaked. I prefer using whole milk and heavy cream because they create a richer, more luxurious custard that keeps the casserole moist as it bakes.

Sprinkle the remaining shredded cheese mixture from Step 1 evenly over the custard layer. Transfer the baking dish to your preheated 350°F oven and bake for 40-45 minutes until the top is golden brown and a knife inserted into the center comes out clean or with just a few moist crumbs. The custard should be set but still slightly creamy, not rubbery.

Remove the casserole from the oven and let it rest for 5-10 minutes—this allows the custard to set slightly and makes serving cleaner and easier. Scatter the sliced chives from Step 1 over the top just before serving for a fresh, bright finish that contrasts beautifully with the rich, savory casserole.