Step 1: Cook potatoes.
Step 2: Meanwhile, in a pan, combine vinegar, sugar and peppercorns. Bring to a boil and stir until sugar is dissolved.
Step 3: Place onion slices in hot vinegar mixture, and let it sit for 5 minutes. Drain onion slices and transfer to plate to cool.
Step 4: Remove flesh off potatoes and transfer to a bowl. Cut into pieces. Add salt and black pepper.
Step 5: Mash potatoes until creamy.
Step 6: Divide potato filling among four tortillas.
Step 7: Top each tortilla with spinach,cheese and onions.