Philly Cheese Steak Zucchini Boats

medium/large wide zucchini ((2 1/2 lbs)*)

olive oil (, divided)

large yellow onion (, sliced into short pieces)

small button mushrooms ((6 oz), sliced)

large red or green bell pepper (, chopped into short slices (or 1/2 of a red 1/2 of a green))

sliced thin deli roast beef (, chopped into small pieces)

Salt and freshly ground black pepper

water or beef broth

shredded provolone cheese

Chopped fresh parsley (, for serving)