Beef Tostadas with Fresh Orange and Chipotle Marinade

orange

chipotle peppers, finely chopped

finely diced onion

brown sugar, packed

olive oil

pepper and salt

flank steak

olive oil spray (or non-stick)

small corn tortillas

Toppings: a few ideas - shredded lettuce, refried beans, grated jack cheese, chopped tomatoes, sour cream, thinly sliced red onions, pickled jalapenos, chopped avocado, cilantro