Pat steak dry with paper towel. Season on both sides with herbes de Provence and salt and pepper. Set aside.
Cook potatoes in a large pan of boiling water, 10 minutes. Add green and broad beans. Cook until all vegetables are just tender, about 5 minutes. Drain; rinse in cold water to cool.
Combine potato, beans, tomato, olives, chives and parsley in a large bowl. Set aside.
Heat ridged cast-iron grill pan or non-stick frying pan over medium-high heat until hot. Grill steak 3 minutes each side or until cooked as liked. Remove to plate; let stand 5 minutes.
Combine oil, vinegar, mustard and 2 tbsp water in a screw-top jar. Season with salt and pepper. Shake to combine.
Slice steak thinly. Add to vegetables. Pour any juices that have collected on the plate into the dressing. Pour dressing over meat and vegetables; toss well. Place spinach and lettuce leaves in a large bowl. Spoon steak salad over the top.