1Microwave frozen green beans 5-6 minutes. Remove from microwave and let cool slightly. (If using fresh green beans, parboil them until tender.)
2Make dressing: Drain olive oil from tuna into a liquid measuring cup. You should have 1/4 cup. Add vinegar, mustard, salt and pepper, and stir with a fork to blend.
3In a large serving bowl, gently combine green beans, cannellini beans, tomato and olives. Break tuna into chunks using a fork and place on top of salad. Drizzle dressing over salad and garnish with parsley, if desired.