Grease your 9x13 inch baking dish well. Spread the cubed bread evenly across the bottom. Make sure the bread covers the entire surface.
Dice the Canadian bacon into small pieces. Scatter it evenly over the bread cubes.
Press down gently to settle everything together.
Crack eggs into your large mixing bowl. Add milk, Dijon mustard, salt, and pepper.
Whisk everything together until completely smooth and combined.
Pour the egg mixture over bread and bacon. Sprinkle Gruyere cheese on top evenly.
Drizzle melted butter over the entire dish. Cover with foil and refrigerate for 2 hours.
Preheat your oven to 375°F before baking. Bake covered for 30 minutes first.
Remove foil and bake 15 minutes more. The top should be golden and bubbly.
Make hollandaise sauce according to package directions. You can also use homemade if preferred. Drizzle it over the baked casserole generously.
Let the casserole rest for 5 minutes. Cut into squares and serve on plates.
Garnish with fresh chopped chives on top.