Heat olive oil in a large skillet over medium-high heat. Place tuna steaks in the hot skillet and sear until lightened but still a little pink in the centers, 3 to 5 minutes on each side. Be careful not to overcook. Remove from heat and allow to cool.
Bring a large pot of lightly salted water to a boil. Add beans and cook for 4 to 5 minutes. Drain and place in a bowl of ice water to "shock" the beans and preserve the green color.
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until they are just a little firm when pierced, 5 to 6 minutes. Drain and place in a bowl of ice water.
Combine garlic, lemon, juice, and lemon zest in a large bowl. Add tuna, potatoes, green beans, and tomatoes. Stir in Italian dressing and coat all ingredients with the dressing. Season with salt and pepper to taste. Chill for at least 2 hours or best overnight.
