Pork Tamales in Banana Leaves (Tamales con Puerco)

1 pound boneless pork shoulder, cut into 1/2-inch cubes
3 cups water
1 small white onion, halved
2 cloves garlic
1 pound fresh tomatillos, husks removed
Boiling water
3 fresh poblano chiles, roasted, peeled, seeded and de-veined
10 small Romaine lettuce leaves
3 sprigs fresh cilantro
3 tablespoons chopped white onions
7 1/2 tablespoons lard or vegetable shortening (at room temperature)
2 cups Masa Harina