Place top oven rack in center position and pre-heat oven to 425°F. Line a large, rimmed baking sheet with parchment paper or a Silpat and set aside.
Combine all dressing ingredients in a glass jar with a tight-fitting lid. Seal and shake vigorously until thoroughly combined. Set aside.
Place the green beans on the prepared baking sheet and drizzle with ½ tablespoon olive oil. Season with salt and black pepper, toss to combine, and spread into a single layer without overcrowding. Set aside.
Place the potatoes in a large saucepan and cover with water by about 2 inches. Generously salt the water and bring to a boil over high heat. Cook for 8–10 minutes, or until fork tender but not mushy. Drain and set aside to cool.
When the oven is ready, place the baking sheet of green beans inside and roast for 5 minutes. Flip the beans and return to the oven for another 5–6 minutes, until crisp-tender. Remove and set aside.
Heat a grill pan over medium-high heat and spray with non-stick cooking spray or brush with a high smoke point oil. Brush the tuna steak on all sides with the remaining 1 tablespoon olive oil and season lightly with salt and pepper.
Place the tuna steak and lemon halves on the hot grill pan. Press down with a spatula for maximum grill marks, if desired. Sear for 2–3 minutes on the first side, then remove the lemon halves and flip the tuna.
Sear the second side for another 2–3 minutes, then quickly sear each edge for 1 minute per side. Remove from heat and set aside.
In a medium bowl, combine the lettuce and tomatoes. Drizzle with a small amount of dressing and toss to combine.
Slice the cooled potatoes into bite-sized pieces and toss with a small amount of dressing. Lightly dress the green beans as well.
To assemble: slice the seared tuna and arrange on a large serving platter. Fill in around it with the dressed lettuce, tomatoes, potatoes, and green beans. Add the sliced hard-boiled eggs, olives, and grilled lemon halves to complete the composition. Season lightly with additional salt and pepper if desired. Serve immediately with remaining dressing on the side. Enjoy!