Mexican Tuna Tostada

1/2 red onion, julienned

12 ounces tuna in water, drained

1 scallion, sliced

1 clove garlic, minced

1 teaspoon chopped oregano leaves

1/4 cup red bell pepper, seeded and diced

1 lime, juiced

1 cup corn kernels, toasted

1 cup romaine lettuce, sliced 1/4-inch thick

1/2 cup nonfat yogurt

1/4 cup capers

1/2 teaspoon chile de arbol, toasted and crumbled

6 tostada shells

6 sprigs fresh cilantro, washed

2 limes, cut into wedges