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 15-20 mins until tender. Cook lentils in one of the smaller saucepan for 10 mins until cooked. Cook green beans for 10 mins in the other smaller saucepan for 10 mins. Lentils and green beans should be cooked but still have a bite. When cooked drain each separately and leave to cool.
In the meantime prepare the rest of the vegetables. Cut baby plum tomatoes in half. Cut yellow pepper and red onion in small dice. Cut artichoke hearts in quarters. Cut cucumber in medium dice.
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 hrs to overnight. Remove from fridge 1/2 hour before serving.