Preheat the oven to 425 degrees. Line a large baking sheet with parchment paper and spread out the potatoes, then string beans, and the white beans, all divided into sections. Add the capers to the white beans. Drizzle everything with olive oil and season with garlic powder, salt and pepper. Toss each section together until coated. Roast for 25 minutes until potatoes are browned, tossing everything halfway through.
While everything roasts, prepare the rest of the salad. For the dressing, whisk all of the ingredients for the vinaigrette together and set aside. Then, halve the tomatoes and set aside.
Once the vegetables and beans are done roasting, assemble your salads. Divide the greens onto plates or bowls. Top each equally with green beans, potatoes, white bean-caper mixture, tomatoes, Kalamata olives, and then drizzle over with dressing. Serve.