Place pork, the 3 cups water, onion halves and 1 clove of the garlic in medium saucepan. Heat over medium heat to boiling; reduce heat to low. Simmer, partially covered, until pork is fork-tender, 20 to 30 minutes. Drain pork, reserving 1 1/2 cups broth.
Place tomatillos and boiling water to cover in medium saucepan. Simmer over medium heat until fork tender, 8 to 10 minutes; drain. Place tomatillos, chiles, 2 small lettuce leaves, cilantro, chopped onion and remaining clove garlic in blender container; process until smooth.
Heat 2 tablespoons lard or vegetable shortening in 4-quart saucepan over medium heat until hot; add tomatillo mixture. Cook and stir until slightly thickened, 3 to 4 minutes. Stir in pork; simmer, stirring occasionally, until pork is very tender, 20 to 30 minutes.
Mix Masa Harina, salt and baking powder in medium bowl. Beat remaining 5 1/2 tablespoons lard in large mixer bowl until light and fluffy, 5 to 10 minutes. Beat in masa mixture, 1/4 cup at a time; beat until thoroughly blended. Heat reserved pork broth jut until warm; gradually beat into masa mixture to form soft, moist dough.
Rinse banana leaves well; using scissors, cut out an discard center rib. Cut leaf halves crosswise into 24 (8-inch) squares. Set stove burner at medium heat. Pass each leaf square quickly across burner a few times until pliable; do not overheat or it will become brittle. Cut remaining 8 small lettuce leaves crosswise into halves.
Spread about 2 tablespoons dough into a 3-inch square on center of 1 banana leaf piece. Top with about 2 tablespoons pork mixture; cover with 1/2 lettuce leaf. Fold sides, then ends of banana leaf over filling to enclose. Repeat to make 16 tamales.
Line large steamer basket with remaining 8 banana leaf squares. Stack tamales sin basket, folded sides down; cover with kitchen towel. Place steamer basket over 3 to 4 inches boiling water; cover with lid. Adjust heat to maintain gentle boil; steam tamales until dough is cooked through, about 1 hour.
Transfer to serving bowl lined with large lettuce leaves; garnish with tomato wedges. Serve hot.