Place a rack in the center of your oven and preheat the oven to 350 degrees F. Coat a deep 9-inch pie dish with nonstick spray.
Prepare any mix-ins.
In a large mixing bowl, whisk together the eggs, milk, half-and-half, salt, pepper, mustard, and nutmeg.
Scatter the mix-ins evenly across the bottom of the prepared pie dish.
Sprinkle the cheese on top.
Carefully pour the egg mixture into the dish. Place the dish on a rimmed baking sheet. Sprinkle the chives over the top.
Bake the quiche on the baking sheet until the center is set, about 35 minutes.
Let cool a few minutes. Cut into big wedges. Enjoy warm.
