Heat an indoor grill, or prepare an outdoor grill with hot coals, or heat a gas grill to hot, or heat an oven broiler.
In a medium-size saucepan of salted boiling water, cook the green beans until crisp-tender, 6 to 7 minutes. Drain the beans; let cool.
In small bowl, whisk together the vinegar, mustard, garlic salt and pepper. Gradually whisk in olive oil until dressing is thick and smooth and well blended.
Season tuna with salt and pepper. Grill or broil 3 minutes. Turn tuna steaks over; grill until cooked through, about 2 minutes. Brush tuna with 2 tablespoons dressing. Set aside.
In large bowl, combine green beans, salad blend, potato slices, green pepper and olives. Drizzle remaining dressing over top; toss to coat all ingredients.
Equally divide the salad among 6 plates. Add 1 piece of tuna to each. Divide hard-cooked egg pieces and tomato slices equally among the plates.