Cut a pocket into the side of the tuna steaks; set aside.
In a bowl, combine olive tapenade, garlic, and capers. Stuff tuna steaks with olive mixture and secure opening with toothpicks. Place stuffed tuna steaks in a large resealable plastic bag with the salad dressing. Squeeze out air and seal bag. Place in second bag to prevent any leaking. Marinate in refrigerator 1 to 3 hours.
Remove tuna from marinade and let come to room temperature, about 20 minutes. Discard marinade.
Heat an oiled grill pan over medium-high heat. Place tuna in grill pan and cook 3 to 4 minutes per side or until done. Be careful not to cook past medium or tuna will dry out quickly. Remove from grill pan and set aside.
Place salad mix onto a large serving platter. Garnish with slices of boiled egg, green beans, tomato wedges, and sliced potatoes. Remove toothpicks from tuna and place on top of greens.