Season the fresh tuna with salt and pepper. Heat a pan with a bit of olive oil over high heat. Sear the tuna for a few minutes on each side, depending on your desired level of doneness.The interior can remain rare to medium-rare. Once seared, remove the tuna from the pan and let it rest for a moment. Then, slice it thinly.
In a small bowl, whisk together the extra-virgin olive oil, lemon juice, Dijon mustard, salt, and freshly ground black pepper until well combined. Adjust the lemon juice, salt, or pepper to taste.
Arrange the mixed greens on a serving platter. Scatter cherry tomatoes, boiled potato slices, hard-boiled egg slices, thinly sliced red onions, and capers over the greens. Place the thinly sliced fresh tuna on top. Drizzle the lemon vinaigrette over the salad. Enoy!
