In a medium bowl, whisk together the eggs, whole milk, salt, and black pepper until fully combined.
Heat olive oil in a non-stick skillet over medium heat. Add diced red bell pepper, green bell pepper, and yellow onion. Sauté for 3-4 minutes until softened.
Pour the egg mixture into the skillet with the sautéed vegetables. Cook, stirring gently, until the eggs are scrambled and fully cooked, about 3-4 minutes. Remove from heat.
Stir in the cooked turkey sausage, shredded cheddar cheese, and shredded Monterey Jack cheese into the scrambled eggs until the cheese is melted and evenly distributed.
Lay one flour tortilla flat on a clean surface. Spread about one-quarter of the egg and sausage mixture evenly over half of the tortilla.
Fold the tortilla in half over the filling to form a half-moon shape. Repeat with the remaining tortillas and filling.
Spray a large non-stick skillet with cooking spray and heat over medium heat.
Place one quesadilla in the skillet and cook for 2-3 minutes on each side, or until the tortilla is golden brown and crispy and the cheese inside is melted.
Remove the quesadilla from the skillet and let it cool completely on a wire rack.
Once cooled, wrap each quesadilla tightly in plastic wrap and then in aluminum foil. Place wrapped quesadillas in a freezer-safe bag or container and freeze for up to 3 months.
To reheat, unwrap the quesadilla and microwave on high for 1-2 minutes or until heated through, or reheat in a preheated 350°F oven for 10-12 minutes until warmed and crispy.