- 2 medium sweet potatoes (1 pound total)
- 1 cup red-wine vinegar
- 1/2 cup sugar
- 4 black peppercorns
- 1 large red onion, sliced
- 1/4 teaspoon kosher salt
- 1/4 teaspoon freshly ground black pepper
- 8 (6-inch) flour tortillas
- 5 cups baby spinach (about 4 ounces)
- 1 cup shredded part-skim mozzarella
- 4 teaspoons olive oil
- Microwave potatoes on high until tender (about 15 minutes).
- Meanwhile, in a medium saucepan, combine vinegar, sugar, and peppercorns; bring to a boil and stir to dissolve sugar. Place onion slices in a medium heatproof bowl; pour hot vinegar mixture over onion and let stand 10 minutes. Drain onion slices and transfer to a plate to cool.
- Split cooked potatoes open with a knife; remove flesh and transfer to a bowl (you should have about 1 cup). Use a fork to mash potato with salt and pepper; divide potato filling among 4 tortillas. Top each with 1 heaping cup spinach, 1/4 cup cheese, and another tortilla.
- Heat a large nonstick skillet over medium-high heat. Working in batches, heat 1 teaspoon oil and transfer 1 quesadilla to skillet; cook until quesadilla is crisp and cheese is melted (about 3 minutes per side). Transfer cooked quesadilla to serving plate. Repeat with remaining oil and quesadillas. Cut quesadillas in quarters and serve each with 1/4 cup pickled onions.