"This is a breakfast casserole created to provide something different from the normal sausage casserole. Even those people that do not like Spam® will be surprised by the taste of these ingredients together."
Preheat oven to 325 degrees F (165 degrees C). Spray a 9x13-inch baking dish with cooking spray.
Cook luncheon meat in a skillet over medium heat until evenly browned; drain. Stir in green onion; reduce heat to low.
Melt butter in a microwave-safe bowl in the microwave, about 30 seconds. Layer croutons on the bottom of prepared baking dish; drizzle melted butter and sprinkle Cheddar cheese over croutons. Spread luncheon meat mixture over Cheddar cheese.
Whisk milk, eggs, mustard, and salt together in a bowl; pour over luncheon meat mixture.
Bake in the preheated oven until golden brown and set, about 40 minutes. Remove from oven and let stand 10 minutes before serving.