Sweet Potato & Spinach Quesadillas

2 medium sweet potatoes
1 cup red wine vinegar
½ cup sugar
4 black peppercorns
1 large red onion, sliced
¼ tsp salt
¼ tsp ground black pepper
8 (6″) flour tortillas
5 cups fresh spinach leaves
1 cup Mozzarella cheese
4 tsp olive oil

1. Microwave potatoes on high until tender (about 15 min, but check regularly).
2. Meanwhile, in a medium saucepan, combine vinegar, sugar, and peppercorns and bring to a boil. Stir until sugar is dissolved. Place onion slices in a bowl, pour hot vinegar mixture over onion and let stand for about 10 minutes.
3. Split cooked potatoes open, remove flesh, and transfer to a bowl. Use a fork to mash the potatoes with salt and pepper, then divide between four tortillas. Top each with 1 heaping cup of spinach, ¼ cup cheese, and another tortilla.
4. Heat 1 tsp olive oil in a nonstick skillet over medium-high heat. Transfer 1 quesadilla to skillet, cook 3 min per side until crisp and browned, then transfer to a serving plate. Repeat with remaining oil and quesadillas.
5. Cut quesadillas into quarters and serve with pickled onion slices.