Make the dressing my whisking together all ingredients except olive oil. Once combined, slowly pour in olive oil while whisking to emulsify oil and create a smooth dressing.

Bring a large pot of water to a boil, then add in the sliced potatoes along with a pinch of salt. Cook potatoes until fork tender, about 10 minutes, then drain from water. Transfer potatoes to a large bowl and toss in ¼ cup of dressing.

Bring another large pot of water to a boil, then add in trimmed green beans. Cook green beans until tender, about 2-3 minutes, then transfer to an ice bath to stop the cooking. Once cool, remove green beans from ice bath.

Assemble all salad ingredients on a large platter – cooked potatoes, green beans, lettuce, tuna, boiled eggs, red onion, tomatoes, radishes, mixed olives, and capers. Drizzle some dressing over salad and serve with extra dressing on the side.