Wipe 2 mushrooms with a damp paper towel, remove stems + scoop out gills; heat 1 tbsp oil in a skillet over med-high + cook mushrooms 2-3 min per side until browned + tender; add salt/pep + set aside

Heat lightly oiled skillet over medium; cook burgers 10 min, flipping every 1-2 min to avoid burning until hot + fully cooked; remove from heat, top with 1-2 tbsp cheese + cover until cheese melts, about 1 min

Place mushrooms on plates bottom side up and add patties; top each with a handful of spinach