Step 1

Melt butter in a large skillet over medium-high heat. Add scallions and garlic; cook, stirring occasionally, until slightly softened, about 2 minutes. Add spinach; cook, stirring often, until starting to wilt, about 1 minute. Remove from heat.

Step 2

Whisk milk and cream cheese in a large bowl until combined. Stir in eggs, mozzarella and salt. Stir in bread cubes and asparagus until well coated. Stir in spinach mixture. Pour into a 9-by-13-inch baking dish coated with cooking spray. Cover with foil; chill for at least 8 hours or up to 24 hours.

Step 3

Preheat oven to 350 degrees F. Bake the casserole, covered, for 30 minutes. Uncover; continue baking until the top is browned and the custard is set, 25 to 30 minutes more.