Bring a large saucepan of lightly salted water to the boil and cook the peas for 2 minutes. Refresh in cold water. Cook the broad beans for 3 minutes, refresh and peel to reveal the bright green inside. Mix the peas and broad beans with the snow pea tendrils and roughly chopped mint.
Make the dressing by whisking the mustard, oil and vinegar. Season to taste with salt and pepper.
Crumble the feta into the salad, carefully mix in the dressing and serve.