Grease a 9x13-inch baking dish with softened butter.
Split the English Muffins and toast them until golden brown. Let them cool and then cut them into 1-inch pieces.
Place the English muffin pieces into the bottom of the buttered baking dish. Top with the Canadian bacon pieces.
MAKE CUSTARD MIXTURE:
In a large bowl, whisk together eggs, milk, onion powder, and garlic until combined.
Pour custard mixture over the English muffins and bacon.
Cover the pan with foil and refrigerate overnight.
Move oven rack to middle position and preheat to 375 degrees F. Take the casserole out of the refrigerator while the oven preheats.
Keep the foil on and bake for 35 minutes. Then, uncover the casserole and continue to bake for 15 minutes more. You’ll know the casserole is done because when a knife is inserted into the casserole, it comes out clean.
MAKE HOLLANDAISE SAUCE:
During the last 15 minutes of baking, make the hollandaise sauce. In a small saucepan, melt the butter over medium heat. While the butter melts, add egg yolks, lemon juice, salt, and paprika to a blender. Blend on medium speed until the mixture is frothy, about 10 seconds. Scrape down the sides of the blender, and blend again on medium speed for 5 seconds.
Turn the blender to the lowest setting and open the pour tube. Slowly drizzle in the hot, melted butter. Continue to blend until the hollandaise sauce thickens and emulsifies about 1-2 minutes.
If the mixture is too thick, then add 1 teaspoon of hot water at a time until the desired consistency is reached.
Season the sauce to taste with salt.
Place slices of casserole onto serving plates, and drizzle with hollandaise sauce. Sprinkle paprika and some chopped chives on top. Serve immediately.
You can prepare the sauce the night before, too. You'll just need to reheat it over low heat until it is warm.