In a large bowl, whisk milk with eggs, spring onions, salt, and onion powder.
Add half the diced bacon to the baking dish, arrange evenly. Top with diced muffins and the other half of bacon. Pour the egg mixture over the bacon and muffins; cover with plastic wrap, put into the fridge and go to sleep.
In the morning, preheat oven to 375°F/190°C/Gas Mark 5.
Remove the casserole from the fridge; sprinkle with paprika.
Cover the baking dish with foil; bake for about 30 minutes or until the eggs are almost set. Remove foil, bake till fully set (about 15 minutes more).
Prepare hollandaise sauce in a saucepan by whisking the mix with 1 cup milk. Add margarine; stir frequently till the mixture is boiling. Reduce heat to low, let simmer and stir till the sauce thickens (1 minute).