Scrub and finely dice the sweet potato. Finely dice the yellow onion. Drain the beans. If using fresh rosemary, chop it finely. If using dried rosemary, grind in the mortar and pestle.

Add all the ingredients to the double-handled pot. Bring to a boil over high heat then reduce heat to medium and simmer, uncovered, stirring occasionally, until the potato and onion are fully softened, about 25 minutes.

Blend using the immersion blender until smooth. Adjust seasoning and stir, then serve.