Brown the sausage: in a skillet over medium-high heat, add ground pork sausage and cook until the sausage is browned. With a spatula, break the meat up while cooking. I like to use a potato masher to break up the meat into even smaller pieces, but if you choose to do this make sure your masher doesn't scratch or hurt your pan. Cooking the meat fully will take 5 to 10 minutes. You're cooking until there's no obvious pink color.
When the meat is fully cooked and broken into small pieces, add 1 teaspoon of flour, stir to mix it in with the meat, and cook for 1 minute.
After a minute of cooking time, add milk, stirring until everything is combined, and allow it to cook until the milk has been absorbed or cooked off; approximately 2 minutes.
Turn the heat off on the stove and add the maple syrup on top of the cooked pork sausage. Stir to combine. Move the sausage out of the pan into a bowl until you're ready for sandwich assembly. Wipe out the pan for use later.
Make the custard: add all French toast custard ingredients to a large bowl. Whisk well to combine everything. Pour the custard into a shallow bowl, flat sheet pan, or pie plate so you can easily dunk a slice of bread into it.
Build the sandwich: on top of one slice of bread add one slice of cheese. Top the cheese with the browned, crumbled, maple pork sausage. Top that with the second slice of cheese (if using). Close off the sandwich with the second slice of bread.
Carefully add the built sandwich to the custard and allow the bottom slice of bread to soak in for at least 30 or 45 seconds.
Add a tablespoon of butter to your wiped-out skillet and turn the heat back on to medium.
Carefully flip the sandwich in the custard and let the second side soak for an additional 30 or 45 seconds.
At this point or sometime very soon, the butter in the pan should be melted and starting to bubble up. Carefully remove the sandwich from the custard and add it on top of the butter in the pan. Cook this first side of the sandwich for 2 minutes, covering the pan with the pan lid or a metal sheet pan.
After 2 minutes, carefully flip the sandwich and cook the second side for another 1.5 minutes, covered. If the top is not brown enough, you can keep flipping the sandwich until both sides get the color you desire.
At this point, the sandwich is done and ready to serve. For best results, the sandwich does benefit from resting on a cooling rack for 2 to 5 minutes to allow the bottom side a chance to dry out and crisp up. This can be done in a very low-preheated oven as well.