Whisk the cranberry sauce, Dijon mustard and cider vinegar until spreadable and refrigerate it.
Mix the turkey, grated onion, panko, egg, salt and sage gently until just combined.
Shape 4 patties with damp hands and chill them for 10 minutes so they firm up.
Coat a skillet lightly with oil and cook the patties over medium heat for 6 to 7 minutes per side.
Check the centers with an instant-read thermometer and continue cooking until every patty reaches 165°F.
Top the patties with cheddar, cover for 1 minute and let the cheese melt while the buns toast.
Spread cranberry mustard on both bun halves and assemble with spinach and one turkey patty.