Place a steamer basket in a saucepan filled with an inch of simmering salted water. Add potatoes to the steamer basket and cook, covered, until potatoes are tender, about 10 minutes. Add zucchini and steam until tender, about 3 minutes. Set aside to cool slightly.
While potatoes and zucchini cook, divide tuna into two portions (including oil) and place half of it in a blender jar. Add yogurt, capers, garlic, vinegar and oil; season with salt and pepper. Blend dressing until smooth.
Transfer warm vegetables to a bowl with onion, tomato, olives, and remaining tuna; toss with dressing to coat. Scatter basil leaves over salad and toss gently.
Before her diagnosis, Caroline wrote a book on cakes called Cake Magic!. She started developing a birthday cake using her gluten-free mix found in that book. Check out other recipes she’s developing for her new life—and the stories behind them—on her blog, The Wright Recipes. Her next book, Catalan Food, written with chef Daniel Olivella, comes out in early September from Clarkson Potter.