In a large bowl, whisk together the eggs, milk, salt, garlic powder, and pepper. Set aside.
Add the olive oil to a large nonstick skillet over medium heat until it is hot and shimmers. Swirl to coat the pan, then add the spinach and cook, stirring often, until it begins to wilt, about 1 minute. Add the beans, reduce the heat to medium low, then carefully pour in the eggs. With a rubber spatula, cook the eggs low and slow, using the spatula to move them around the pan often. Continue cooking until the eggs are scrambled and just set, about 5 minutes. Taste and season with additional salt or pepper as desired. Remove from the heat. (If freezing the quesadillas, let the filling cool completely.)
To assemble the quesadillas: Sprinkle a tortilla with one-eighth of the shredded cheese, leaving a small border all the way around the edge. Spoon one-eighth of the egg mixture on top, then fold the tortilla in half. Repeat with the remaining tortillas.
To cook immediately: Carefully wipe out the skillet. Increase the heat to medium and lightly coat the skillet with nonstick spray (or brush with a bit of additional olive oil). Cook the assembled quesadillas on both sides until golden and the cheese is melted, about 5 to 6 minutes total. Cut into wedges and serve warm.
To freeze: Let the egg filling cool completely to room temperature. Once cooled, form the quesadillas as directed above, then wrap each assembled quesadilla individually in plastic wrap. Arrange the quesadillas in a single layer on a baking sheet or similar flat surface that will fit in your freezer. Place the sheet in the freezer until the quesadillas are firm, then transfer them to a freezer bag or airtight container. Freeze for up to 2 months. To cook from frozen, remove the plastic wrap and warm the quesadilla in the microwave for 2 to 3 minutes until heated through. Alternatively, you can let them thaw overnight in the refrigerator and cook in a skillet as directed above.