Boil the pasta in a large pan of water according to the pack instructions. Add the peas and broad beans (individually shelled, if preferred) to the pan and simmer for around 2 minutes.
Meanwhile put the crème fraîche and the vegetable stock in a small saucepan. Place over a gentle heat for 5 minutes, stirring occasionally and making sure it doesn't boil. Season, take off the heat and stir in the mint.
Drain the peas and pasta and return to the pan. Stir the crème fraîche sauce through the pasta and serve immediately.