Roast the poblano peppers over an open flame or under the broiler until blistered on all sides. Transfer to a bowl, cover, and let steam for 10 minutes.
Peel off the skins, remove stems and seeds, then slice the peppers into thin rajas.
Heat oil in a skillet over medium heat and sauté the onion until soft. Add the rajas and salt, cooking 3–4 minutes. Remove from heat and stir in the cheese. Set aside.
In a large bowl, mix masa harina, baking powder, and salt.
Beat the shortening until light and fluffy, then mix into the masa.
Gradually add warm broth until the dough is soft, smooth, and spreadable.
Drain corn husks and pat dry. Spread about ¼ cup masa onto each husk, leaving space at the bottom.
Add 2 tablespoons of rajas filling to the center, then fold the sides and bottom of the husk to enclose.
Stand tamales upright in a steamer with the open ends facing up.
Steam over medium heat for 60 minutes, checking water level as needed.
Tamales are done when the masa pulls cleanly away from the husk. Let rest 10 minutes before serving.