Spray a 8x11 or 9x9 baking dish with non-stick cooking spray. set aside.

In a large skillet heat olive oil over medium heat. Add onions and garlic and cook until onions are just about translucent ( about 2-3 min.) . Add Canadian bacon and cook for an additional minute. Add spinach and stir into mixture. Turn off heat and season with salt and pepper (to taste). Set aside.

In a medium bowl wisk together eggs, milk and dry mustard. Season with salt and pepper to taste.

Spread half of the bread cubes in the bottom of prepared baking dish. Spoon half the bacon/spinach mixture over the top of bread. Sprinkle half the cheese on top. Repeat by spreading the remaining bread crumbs, topped with the remaining bacon/spinach mixture. Pour the egg mixture over the casserole, making sure to get the sides of the dish. Top with remaining cheese. Cover and refrigerate for at least 2 hours (or overnight).

Preheat oven to 400F. Uncover dish and bake until golden brown (about 45-50 minutes) . If necessary cover casserole with aluminum foil for last few minutes of baking if it is becoming too brown. Remove from oven and let sit 5 minutes before serving