Place a large non-stick skillet over medium heat.
Add the ground breakfast sausage to the skillet.
Cook and crumble the sausage until it is fully cooked and no longer pink.
Transfer the cooked sausage to a paper towel-lined bowl to drain excess grease.
Wipe the skillet clean.
Place the skillet over medium-high heat.
Add 2 tbsp unsalted butter to the skillet and allow it to melt.
Add the beaten eggs to the skillet.
Cook the eggs, stirring gently, until they are softly scrambled and almost fully set.
Remove the skillet from the heat.
Remove the paper towel from the bowl of cooked sausage.
Add the scrambled eggs to the cooked sausage.
Stir gently until the sausage and eggs are evenly combined.
Place the skillet over medium heat.
Spread the remaining 2 tbsp softened unsalted butter on one side of each slice of sourdough bread.
Place one slice of bread in the skillet with the buttered side facing down.
Place one slice of Colby-Jack cheese on top of the bread.
Add approximately 1/2 cup of the sausage and egg mixture over the cheese.
Place one additional slice of Colby-Jack cheese over the sausage and egg mixture.
Place a second slice of bread on top, with the buttered side facing upward.
Cook for 3 to 4 minutes, or until the bottom slice of bread is golden brown.
Carefully flip the sandwich with a wide spatula.
Cook the second side for 3 to 4 minutes, or until golden brown and the cheese is fully melted.
Remove the sandwich from the skillet.
Cut the sandwich in half, if desired.
Repeat the process with the remaining bread, cheese, and sausage-egg mixture.
Serve immediately while warm.