Put peppers on a baking pan and broil 3 in. from heat, turning as needed until all sides are charred, 10 to 12 minutes. Cool; remove stems, seeds, and skin. Coarsely chop peppers.
In a large saucepan over medium-high heat, cook onion in 2 tbsp. oil, stirring, until lightly browned, 6 to 8 minutes. Add potatoes, peppers, broth, and salt. Cover and bring to a boil; reduce heat and simmer until potatoes are tender when pierced, 10 to 15 minutes.
In a blender, purée soup in batches, holding lid down with a towel. Pour back into saucepan and thin to desired consistency with more broth if you like; reheat until hot. Drizzle each serving with a scant 1 tsp. oil.
Note: Nutritional analysis is per serving.