Brown the sausage in a skillet over medium heat, using a wooden spoon to break it up into pieces. Drain off excess fat and transfer the sausage to a large mixing bowl.

In the same skillet over medium heat, add the oil and sauté the onion until translucent, 5 to 6 minutes. Stir in the hash browns and cook undisturbed until lightly browned underneath, about 5 minutes. With a spatula, flip the mixture over and cook 5 minutes longer.

Add this hash brown mixture to the sausage. Add the grated cheese and bread cubes and stir together thoroughly.

In another mixing bowl, whisk together the eggs, milk, syrup, mustard powder, salt, and Worcestershire sauce. Pour over the sausage-potato mixture. Cover and refrigerate 8 hours or overnight. In the morning, leave at room temperature for 30 minutes while preheating the oven to 350°F. Spoon the mixture into a lightly buttered casserole dish and bake until bubbling and browned, 40 to 45 minutes. Let stand at room temperature for 5 minutes before serving.