Heat up some salted water in one large saucepan and 2 smaller ones. When the water is boiling cook potatoes in the large saucepan for 15 to 20 minutes until tender. Cook lentils in one of the smaller saucepans for 10 minutes until tender. Cook green beans for 10 minutes in the other smaller saucepan for 10 minutes. When cooked, lentils and green beans should be tender but still have a bite. Drain each separately and leave to cool.
Place cooled lentils, potatoes and beans in a large salad bowl. Add tomatoes, pepper, cucumber, onion, olives and artichokes followed by finely chopped basil.
Make vinaigrette in a separate bowl by mixing mustard, olive oil and vinegar until you have a thick dressing.
Toss salad and dressing together. Season to taste.
Leave covered in the fridge for 2 hours or overnight. Remove from fridge 1/2 hour before serving to take the chill off. Give a toss to refresh and serve.