Preheat your oven to 375°F (190°C). Begin by cubing your English muffins and arranging them on a baking sheet. Toast them in the oven for 8-10 minutes, or until they turn lightly golden and crisp.

In a medium saucepan, whisk together whole milk, all-purpose flour, melted unsalted butter, fresh lemon juice, Dijon mustard, and a pinch of salt. Cook over medium heat for about 5-6 minutes, stirring constantly until the mixture thickens to a custard-like consistency. Once thickened, stir in the shredded sharp cheddar and Gruyère cheese until melted and smooth.

In a greased 9x13-inch baking dish, start layering your toasted muffin cubes evenly across the bottom. Scatter the crispy Canadian bacon over the muffins, ensuring a balanced distribution. Carefully pour the creamy custard mixture over the layers, making sure every piece of muffin and bacon is well-coated in the rich sauce.

Crack whole eggs gently over the casserole, spreading them evenly across the surface. Make sure to space them apart to allow for even cooking.

Place the baking dish in the preheated oven and bake for 30-35 minutes or until the eggs are set and the top is beautifully golden brown.

After baking, allow the casserole to cool for about 5 minutes before slicing into squares. Garnish each piece with freshly chopped chives.