Bring water to a boil in a pot.
Cook the green beans for 2-3 minutes. They should still be crispy.
Drain the beans.
Arrange all the vegetables on a big plate.
Combine dijon mustard, lemon juice, balsamic vinegar in a bowl. Add some water if needed. Set aside.
Preheat a grill pan over medium/high heat with some olive oil.
Season the tuna steak/steaks with salt and pepper.
Grill for 1/2-2 minutes on each side, depending on how thick your tuna is and how raw you like it.
Slice the tuna stake in 1/2 cm thick slices and put on top of the salad.
Serve with the dressing.