Place top oven rack in the center position and pre-heat oven to 425°F. Line a large, rimmed baking sheet with parchment paper or a Silpat™ baking sheet and set aside.
Combine the dressing ingredients in a large glass jar with a tight-fitting lid. Seal tightly and shake vigorously until thoroughly combined. Set aside.
Place the green beans on the prepared baking sheet and drizzle with one-half tablespoon olive oil. Season with salt and black pepper, to taste, and toss to combine before spreading into a single layer on the baking sheet without overcrowding. Set aside.
Place the potatoes in a large saucepan or pot and fill with water until they are covered by approximately 2 inches. Generously salt the water and bring to a boil over high heat for 8-10 minutes, or just until the potatoes are fork-tender, but not mushy. Do not overcook. Drain the potatoes and set them aside to cool.
When the oven is ready, place the baking sheet with the green beans inside for 5 minutes. Flip the beans and return to the oven for another 5-6 minutes, or just until crisp-tender. Do not overcook. Remove from oven and set aside.
Spray a large grill pan with non-stick cooking spray (or brush with a neutral oil with a high smoke point) and set over medium-high heat. Brush the tuna steak on all sides with the remaining olive oil and lightly season with salt and black pepper, to taste.
Place the tuna steak and lemon halves on the hot grill pan. (If desired, press down on the tuna and lemon halves with a spatula to maximize the grill lines). Sear the tuna for approximately 2-3 minutes on one side before removing the lemon halves and flipping the tuna.
Repeat on the remaining side for another 2-3 minutes, then quickly sear each edge for another 1 minute per side. Remove from heat and set aside.
Combine the lettuce and tomatoes in a medium bowl and drizzle a small amount of the dressing on top. Toss to combine and set aside.
Slice the potatoes into bite-sized pieces and drizzle a small amount of the dressing on top. Toss to combine. Lightly dress the green beans with some dressing, as well, and toss to combine. Set aside.
To assemble the salad, slice the seared tuna, and arrange it on a large serving platter. Fill in around it with the lightly dressed lettuce, tomatoes, potatoes, and green beans. Add the sliced hard-boiled eggs, olives, and grilled lemons to complete the composition. Season lightly with additional salt and black pepper, if desired. Serve immediately with the remaining dressing on the side. Enjoy!