1 small red onion, thinly sliced
1/4 cup balsamic vinegar
4 ten-inch whole wheat tortillas
1 cup shredded sharp cheddar cheese
8 slices deli-turkey, preferably smoked (roughly 8-oz of turkey)
Combine the onion and vinegar in a bowl; let marinate for 5 minutes. Drain. (You can reserve the vinegar for another use, such as salad dressing, if you’d like).
Warm 2 tortillas in a large nonstick skillet over a medium-high heat for about 45 seconds, then flip. Pull the tortillas up to the edges of the pan so that they are not overlapping.
Working on one half of each tortilla, sprinkle one-fourth of the cheese, cover with 2 slices of turkey, and top with one-fourth of the onions. Fold the tortillas in half, flatten gently with a spatula, and cook until the cheese starts to melt, about 2 minutes.
Flip and cook until the second side is golden, 1 to 2 minutes more. Transfer to a plate and cover to keep warm. Make 2 more quesadillas with the remaining ingredients.