1. Brown the lamb:
Heat 1 Tbsp olive oil in a large soup pot over medium-high heat. Season the American lamb with salt and pepper. Add to the pot and cook until browned on several sides, about 5–6 minutes. Transfer to a plate.     

2. Build the base:
Reduce heat to medium and add the remaining 1 Tbsp olive oil. Add the onion and celery with a pinch of salt. Cook, stirring occasionally and scraping up browned bits, until very soft, 6–8 minutes. Stir in the garlic and cook until fragrant, about 1 minute.

3. Simmer the lamb:
Return the lamb and any juices to the pot. Add the chicken stock and bring to a boil. Reduce to a gentle simmer, partially cover, and cook until the lamb is tender, 45–55 minutes. If any bubbles or foam comes to the top, skim it off with a spoon. 

4. Add beans and greens:
Stir in the oregano, cannellini beans, and simmer for 5 minutes. Add the greens and cook just until wilted. Then add the peas, and asparagus and simmer for 2 minutes before removing from the heat. 

5. Brighten the soup:
Stir in the lemon zest and juice and taste for seasoning, adding more salt and pepper if needed.

6. Make the lemon herb swirl:
In a small bowl stir together the parsley, mint or dill, garlic, lemon zest and juice. Drizzle the olive oil into the bowl while stirring to incorporate, and season with salt to taste. 

7. Serve:
Ladle the soup into bowls and swirl a spoonful of the lemon herb sauce over the top.