In a medium bowl, whisk together olive oil, lime juice, minced garlic, cumin, chili powder, salt and pepper. Marinate the flank steak for 30 minutes to 2 hours.

Preheat grill or skillet. Cook steak for 4-6 minutes on each side until desired doneness. Let rest before slicing.

In a bowl, combine diced tomatoes, red onion, jalapeño, lime juice, and salt to prepare fresh salsa.

Layer sliced steak, lettuce, avocado, and salsa on tostada shells.

Garnish with cilantro and serve with lime wedges.