2 medium sweet potatoes (1 pound total), cubed
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 spinach (or your favorite tender cooking green)
1 cup shredded part-skim mozzarella
4 teaspoons olive oil
Bring a large pot of salted water to a boil. Add cubed potatoes and cook until tender, 20 to 30 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.
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 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.