Heat 1 tsp oil in skillet over med + thinly slice ½ of onion; sauté 8-10 min until browned, season with salt + add 2 tbsp water; cover + cook 3-5 min until softened; set aside

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

Meanwhile, toast 2 buns until warm + golden; add burgers to buns + top with onions + a handful of spinach