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 with 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 pink remains + cheese has melted

Toast 2 buns until warm + golden; add burgers to buns, top with onions + a handful of spinach; toss remaining spinach with 2 tbsp vinegar or lemon juice, 1 tbsp oil + salt/pep for a quick side salad—looks great!