Step 1Cook the green beans in boiling salted water for 3 minutes. Add the peas and cook for 3 more minutes. Drain and transfer to a bowl. While the vegetables are still hot, add the celeriac and mix well.
Step 2In a small bowl, combine the olive oil, vinegar and mustard. Season with salt and sugar to taste; mix well.
Step 3Pour the dressing over the beans, peas and celeriac. Toss. Add the mango, mix gently and serve.