On a gas stove, turn a burner to high and set the poblano directly over the flame, turning it with tongs, until completely charred, 5 to 8 minutes. Alternatively, on an electric stove, heat the broiler on high and char the poblano on all sides on a baking sheet placed directly under the broiler. Put the poblano in a bowl, cover, and set aside to steam and loosen the skin. When cool enough to handle, peel, seed, and cut the poblano into 1/4 -inch dice.
Heat an 11- to 12-inch cast-iron skillet over medium-high heat, add the onion and cook, turning a few times, until soft and browned in places, about 5 minutes.
In a blender, purée until smooth all but 1 Tbs. of the poblano, the onion, broth, avocado, cilantro, parsley, marjoram or oregano (if using), 2 Tbs. of the lime juice, cumin, coriander, and 1 tsp. salt. Blend in the yogurt. Season to taste with more salt. Chill well.
Heat the oil in a 10-inch skillet over medium heat. Add the pepitas and cook until they begin to pop and color a bit, about 3 minutes. Add the remaining 2 Tbs. lime juice, the chile powder, and 1/8 tsp. salt and stir until the juice has evaporated, leaving a film on the pan.
Season the soup to taste with salt and thin with broth if necessary. Divide among 6 cups or small bowls and garnish with the pepitas, the remaining poblano, and a few pinches of chile powder.