The night before, whisk together the flour, milk, and sourdough starter until completely smooth with no lumps.
Cover tightly and refrigerate for 8 to 12 hours. If using sausage, cook it now, cover, and refrigerate alongside the batter.
The next moring, Preheat your oven to 425F (220C).
Remove the fermented batter and sausage from the refrigerator and let sit at room temperature while the oven preheats.
Add the fermented batter, eggs, maple syrup, and salt to a blender. Blend for 30 seconds until very smooth.
Once the oven is fully preheated, place the butter in your 13x9 glass baking dish and put it in the oven for 6 to 8 minutes until melted and sizzling. Do not leave it longer or the butter will burn.
Carefully remove the hot dish from the oven. Tilt and rotate to coat the entire bottom and sides with butter. Place back on the oven rack.
Slowly pour the batter into the center of the hot buttered dish. It will sizzle. Gently push the rack bac into the oven and close.
Bake for 15 minutes until the batter begins to puff and set.
Working quickly, open the oven and evenly sprinkle the shredded cheese over the surface, then add the crumbled sausage.
Continue baking for 8 to 10 minutes until deeply golden, puffed around the edges, and the cheese is fully melted.
Remove from oven and serve immediately while hot and puffed.