Whisk together the eggs and milk until well combined in a large bowl.
Fold in the frozen cauliflower, green onion, cheese, salt, and pepper until well distributed.
Pour the mixture into a well-seasoned cast iron skillet (we used a 10-inch skillet) or a lightly greased oven-safe pan/baking dish.
Bake for 45-50 minutes, or until the center of the frittata looks set in the middle and the edges are golden brown.
Let this cool at least 5-10 minutes before slicing and serving. This also keeps well in the fridge for about 1-2 days and can be reheated in the oven or microwave.