Heat 1 tbsp oil in skillet over med + thinly slice onion; sauté 8-10 min until browned, season with salt + add 2 tbsp water; cover + cook 3-5 min until softened (or grill ¼ to ½-inch sliced rounds 5 min each side until charred + slightly softened); set aside

Heat lightly oiled large skillet over high; cook 4 burgers with salt/pep 3 min one side; flip over, cook 2 min, then add 1 slice of cheese to each burger; cover pan + cook 1-2 more min until cooked through, no pink remains + cheese has melted

Meanwhile, toast 4 buns until warm + golden; add burgers to buns, top with onions + a handful of spinach; toss remaining spinach with 2 tbsp vinegar, 1 tbsp oil + salt/pep for a quick side salad—delish!