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

Wipe 2 mushrooms with a damp paper towel, remove stems + scoop out gills; heat 1 tbsp oil in skillet over med-high + cook shrooms 2-3 min per side til browned + tender; add salt/pep, ½ of seasoning blend + top each shroom with 1-2 tbsp cheese; remove pan from heat + cover 2-3 min til cheese melts

Meanwhile, toast 2 buns til warm + golden

Top buns with shrooms, caramelized onions + handful of arugula; close the lid (toss leftover arugula with ½ seasoning blend, 2 tbsp water, 2 tsp oil + salt/pep)