Preheat the oven to 450°F. Spread croissants on a large baking sheet cut side up and bake until golden brown, about 6 minutes.
In a large skillet over medium heat, cook the bacon until crispy, about 8 minutes. Drain on paper towels, then chop into bite-size pieces.
In a large bowl, whisk together the eggs, milk, heavy cream, chives, and thyme. Season with salt, pepper, and cayenne, then stir in about ¾ cup Gruyère and ¾ cup mozzarella.
Lightly butter a 9×13 baking dish. Scatter the croissants and cooked bacon into the dish, then pour the egg mixture on top. Press down gently so the croissants soak. (Optional: refrigerate 30 minutes up to overnight.)
Preheat the oven to 350°F. Top the casserole with the remaining cheeses. Bake until golden, about 45 minutes. Garnish with more chives and serve warm.
From Delish (@delish on Instagram, recipe by Lauren Miyashiro). Make-ahead friendly – assemble the night before.
