1 pound Yukon Gold potatoes, peeled and cut into 2-inch chunks
1 large carrot, cut into 1/2-inch dice
1/2 pound cooked lamb, cut into bite-size pieces
2 cups frozen lima beans (10 ounces)
2 tablespoons unsalted butter, at room temperature
1 1/2 teaspoons harissa (see Note)
1 teaspoon chopped rosemary
In a large saucepan, heat the olive oil. Add the onion, garlic and celery, season with salt and pepper and cook over moderately low heat, stirring occasionally, until the vegetables are softened, about 7 minutes. Add the tomato paste and cook over moderately high heat, stirring, until glossy, about 1 minute. Add the stock and bring to a boil. Add the potatoes and carrot, cover and simmer over low heat until the potatoes are tender, about 20 minutes.
With a slotted spoon, transfer the potatoes to a large, shallow bowl. Add the lamb and lima beans to the soup; cover and simmer over low heat until the lima beans are tender, about 3 minutes.
Using a fork, mash the potatoes thoroughly, then blend in the butter, harissa and rosemary. Stir the mashed potatoes into the soup, season with salt and pepper and serve.
The soup can be refrigerated for up to 2 days. Reheat gently.
Harissa can be found in Middle Eastern markets and specialty food stores.