Set an oven rack about 6 inches from the broiler and heat the broiler to high.

Put the poblano halves cut side down on a sheet pan. Brush them with 1 tablespoon of the olive oil.

Broil the poblanos for 8 to 10 minutes, until the skins are blistered and black in spots.

Transfer the poblanos to a bowl and cover the bowl with a plate or clean towel. Let them sit for 5 minutes so the skins loosen.

Set a large soup pot on medium heat and add the remaining 1 tablespoon olive oil.

Put the sliced onion and 1/2 teaspoon salt into the pot. Cook for 4 to 5 minutes, stirring often, until the onion is soft and glossy.

Scoop about half of the cooked onion from the pot into a blender.

Peel off the loose charred skins from the poblanos. Put the peeled poblanos into the blender with 1 cup of the vegetable broth.

Blend until the poblano mixture is smooth.

Pour the blended poblano mixture back into the soup pot with the remaining onion.

Add the remaining 5 cups vegetable broth, the black beans, and the cumin to the pot. Increase the heat to medium-high and bring the soup to a gentle boil.

Put the zucchini into the pot. Cook for 3 minutes, until it starts to soften but still keeps its shape.