Toast 2 buns until warm + golden; thinly slice ½ (or whole) onion

Heat lightly oiled large skillet over high; cook burgers with salt/pep 3 min on one side; flip over, cook 2 min, then add 2 slices of cheese to each burger; cover pan + cook 1-2 more min until cooked through, no pink remains + cheese has melted; set burgers aside to rest + heat 1 tbsp oil in the same skillet; sauté onions with salt/pep 3-4 min until starting to soften + brown (scraping up any brown bits from the pan as they cook for extra flavor)

Add burgers to buns with onions + a handful of spinach; drizzle balsamic on each—looks tasty!