First, prepare your 9x13-inch baking dish. Grease it thoroughly with butter or a non-stick cooking spray. This important first step ensures that your casserole won't stick to the bottom, making it much easier to serve clean slices later on.
In a large bowl, whisk together the 8 eggs, 2 cups of milk, the chopped green onions, and the teaspoon of onion powder and salt. Continue whisking until the mixture is smooth and well-combined. This egg custard is what will bind the casserole together.
Now it's time to build your casserole. Spread the cubed English muffins evenly across the bottom of your greased baking dish. Sprinkle the chopped Canadian bacon over the muffins. Finally, pour the egg mixture evenly over the top, making sure all the muffins are saturated.
Cover the baking dish tightly with foil or plastic wrap. Place it in the refrigerator to chill for at least 8 hours, or preferably overnight. This chilling period is crucial; it allows the English muffins to soak up the egg custard, ensuring a perfect texture.
When you're ready to bake, preheat your oven to 375°F (190°C). Remove the casserole from the refrigerator, uncover it, and sprinkle the paprika over the top. Cover it again with foil and bake for 30 minutes. Then, remove the foil and bake for another 15 minutes, or until the center is set and the top is golden brown.
While the casserole is baking, prepare the Hollandaise sauce. In a small saucepan, whisk together the remaining 1 cup of milk and the Hollandaise sauce mix. Add the ¼ cup of butter and cook over medium heat, whisking constantly until the sauce thickens, which should take about a minute.
Let the casserole rest for a few minutes after taking it out of the oven. This will help it set for easier slicing. Cut it into 8 or 10 equal portions, place them on plates, and drizzle the warm, creamy Hollandaise sauce over each serving before enjoying.