Place the fingerling potatoes in a pot of cold salted water, bring to a boil, and cook until tender, about 10-12 minutes.
Add the green beans to the boiling potato water for the final 3 minutes of cooking, then drain both and immediately shock in ice water to preserve the vibrant color.
In a separate small pot, boil the egg for exactly 7 minutes for a jammy yolk, then transfer to an ice bath, peel, and slice in half.
Prepare the vinaigrette by whisking together 0.5 tbsp of the olive oil, lemon juice, Dijon mustard, and capers in a small bowl.
Pat the tuna steak dry and season both sides with sea salt and black pepper.
Heat the remaining 0.5 tbsp of olive oil in a cast-iron skillet over high heat and sear the tuna for 1-2 minutes per side until golden on the outside but rare in the center.
Slice the tuna against the grain into half-inch thick pieces.
Assemble the salad by placing the mixed greens on a large plate and topping with the potatoes, green beans, cherry tomatoes, olives, egg, and seared tuna.
Drizzle the lemon-caper vinaigrette over the entire salad and serve immediately.