Heat a large skillet over medium-high heat. Add the breakfast sausage and cook, breaking it up into small crumbles with a spatula, until browned and cooked through (about 5-7 minutes). Drain any excess grease and transfer the sausage to a bowl.

In a medium mixing bowl, whisk together the eggs, milk, salt, and pepper until well combined and slightly frothy.

Return the skillet to medium-low heat and melt the butter. Pour in the egg mixture. Cook gently, stirring frequently with a silicone spatula, until soft curds form and the eggs are just set but still slightly moist. Remove from heat and gently fold in the cooked sausage. Let the mixture cool for 10 minutes.

Lay the tortillas flat on a clean work surface. Sprinkle about 2 tablespoons of shredded cheese on one half of each tortilla. Top with a generous scoop (about 1/2 cup) of the egg and sausage mixture. Sprinkle another 2 tablespoons of cheese over the filling, then fold the empty half of the tortilla over to create a half-moon shape.

Wipe the skillet clean, lightly coat with cooking spray or a little oil, and place over medium heat. Toast the assembled quesadillas for 1-2 minutes per side until golden brown and crispy, and the cheese is melted. Cook in batches to avoid overcrowding the pan.

Allow the toasted quesadillas to cool completely on a wire rack. This prevents condensation from making them soggy in the freezer.

Once fully cooled, wrap each quesadilla tightly in plastic wrap or aluminum foil. Place the wrapped quesadillas into a large zip-top freezer bag. Squeeze out as much air as possible, seal, label with the date, and freeze.