In a medium bowl, whisk together the eggs, whole milk, salt, and black pepper until well combined.
Heat a large skillet over medium heat and add the breakfast sausage. Cook, breaking it apart with a spatula, until browned and cooked through, about 5-6 minutes. Remove the sausage from the skillet and set aside.
In the same skillet, melt the unsalted butter over medium heat. Pour in the egg mixture and cook, stirring gently, until the eggs are scrambled and just set, about 3-4 minutes. Remove from heat.
Lay one flour tortilla flat on a clean surface. Evenly distribute one-quarter of the scrambled eggs over half of the tortilla.
Top the eggs with one-quarter of the cooked sausage and one-half cup of shredded cheddar cheese.
Fold the tortilla in half over the filling to form a half-moon shape and gently press down.
Repeat the filling and folding process with the remaining tortillas and ingredients.
Heat a clean skillet or griddle over medium heat and lightly spray with non-stick cooking spray.
Place one quesadilla in the skillet and cook for 2-3 minutes on each side, or until the tortilla is golden brown and the cheese is melted. Repeat with remaining quesadillas.
Allow the quesadillas to cool completely on a wire rack.
Wrap each quesadilla tightly in plastic wrap, then place them in a freezer-safe zip-top bag or airtight container.
To reheat, unwrap a frozen quesadilla and microwave on high for 1-2 minutes or until heated through, or reheat in a skillet over medium heat for 3-4 minutes per side.