Prepare the Baking Dish: Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish with cooking spray or butter to prevent sticking and ensure easy serving later.
Make the Batter: In a large mixing bowl, whisk together the eggs, milk, pancake mix, maple syrup, baking powder, salt, and pepper until well combined. The mixture should be smooth but not overmixed.
Add Fillings: Stir in 1 cup of the shredded cheddar cheese and half of the cooked, crumbled breakfast sausage. Mix gently until the ingredients are evenly distributed throughout the batter.
Layer the Casserole: Pour half of the batter into the prepared baking dish, spreading it evenly. Sprinkle the remaining cooked sausage over this layer, then pour the remaining batter on top, creating a sandwich effect with the sausage in the middle.
Bake the Casserole: Place the baking dish in the preheated oven and bake for approximately 30 minutes, or until the casserole is set in the middle and the edges begin to turn golden brown.
Add Cheese Topping: Remove the casserole from the oven and sprinkle the remaining 1/2 cup of shredded cheddar cheese over the top. Return to the oven for an additional 3-5 minutes, until the cheese is completely melted and slightly bubbling.
Serve: Allow the casserole to cool for about 5 minutes before cutting into squares and serving. This brief rest helps the casserole set up for easier serving.