Preheat an oven, then grease a 9×13 baking dish with nonstick cook spray.
Heat a skillet over medium heat. When hot, add bacon then cook until crispy, stirring often. Use a slotted spoon to transfer the bacon to the prepared dish.
Combine 1 cup of shredded Gruyere and shredded Cheddar cheese in a small bowl. Sprinkle about half of the cheese on top of the bacon, then reserve the rest. Place the ripped croissants on top.
In a large mixing bowl, whisk together large eggs, whole milk, heavy cream, chopped sage, Dijon mustard, Kosher salt, and black pepper. Pour the egg mixture over the croissants, then sprinkle the remaining cheese on top.
Bake until the croissants are golden brown and the cheese has crisped up. Check 30 minutes into cook time – if the croissants are browning too quickly, you can cover with aluminum foil. Cool, then garnish with chives and enjoy immediately.