Flank Steak Tostadas

Preheat a gas or charcoal grill to medium.

Combine onion and vinegar in a small bowl. Let stand until ready to serve.

Combine Southwest seasoning and salt in another small bowl. Rub the mixture all over steak with your fingers.

Oil the grill rack (see Tip). Place the steak on the rack directly over the heat; cover and grill, turning once, until desired doneness (145 degrees F for medium), 13 to 16 minutes. Transfer to a cutting board and cover loosely with foil; let stand for 10 minutes.

Meanwhile, stir yogurt and lime juice in a small bowl.

Thinly slice the steak across the grain. Drain the onion; discard the vinegar, or reserve for another use (see Tip). Divide lettuce among tostada shells. Top with the steak, tomatoes, the pickled onion, cilantro and jalapeño. Drizzle with the yogurt mixture. Serve with lime wedges.