Watermelon Pork Tacos in Lettuce Shells

For the pork:

2 pork tenderloins (about 2 pounds), silver skin trimmed, cut into 1-inch-thick slices

1 tablespoon onion powder

1 teaspoon red pepper flakes

1 tablespoon freshly ground pepper

2 tablespoons chopped fresh ginger

1 tablespoon seeded and minced serrano chile

2 tablespoons minced fresh garlic

3 tablespoons soy sauce

2 tablespoons oyster sauce

1/2 teaspoon toasted sesame oil

For the salsa:

2 tablespoons rice vinegar

1/4 cup olive oil

1 tablespoon soy sauce

1/4 teaspoon toasted sesame oil

1 cup small (about 1/4 inch)watermelon cubes

1/4 English cucumber, seeded and cut into 1/4-inch cubes

1 Hass avocado, pitted, peeled and cut into 1/4-inch cubes

4 large, sturdy romaine-heart

leaves, cut into 4-inch lengths (to resemble taco shells)