Prepare pan: Butter a 9x13-inch baking dish generously.

Layer the strata: Spread half of the torn croissant pieces in the bottom of the prepared dish. Layer with half the ham, half the Gruyère (1 cup), and half the baby greens. Repeat with remaining croissants, ham, 3/4 cup cheese, and greens. Reserve 1/4 cup cheese for topping.

Make custard: In a large bowl, whisk together eggs, milk, cream, Dijon mustard, salt, pepper, and nutmeg (if using) until well combined.

Pour and rest: Pour the egg mixture evenly over the layered ingredients, pressing down gently to ensure the croissants absorb the liquid. Sprinkle the reserved 1/4 cup cheese on top.

To bake now: Cover the dish with foil and let sit for 15-20 minutes while you preheat the oven to 350°F (175°C). This allows the croissants to soak up the custard.

To bake later: Cover tightly with plastic wrap or foil and refrigerate for up to 24 hours. The longer it sits, the better the flavors meld.

Bake: Preheat oven to 350°F (175°C). Place the covered baking dish on the center rack and bake for 35 minutes. Remove the foil and bake for an additional 15 minutes, until the top is golden and the center is set (a knife inserted in the center should come out mostly clean).

Rest and serve: Let the strata rest for 5-10 minutes before slicing. Garnish with fresh chives if desired.