Cover the bones with water and bring to a simmer until reduced to 1/3, for about 2 hours.
- Add the shallots, garlic, prosciutto, and mushrooms and cook for 20 minutes.
- Strain the stock through a cheesecloth and discard the solids.
- Heat the brodo in a saucepan and add the green beans. Simmer for 5 minutes and add the mushrooms and lemon juice.