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)