For the vinaigrette, combine the lemon zest and juice in a large bowl. Add anchovy, if using, and mix well. Slowly add oil in a steady stream until incorporated; season generously with salt and pepper.
Place potatoes and garlic in a small pot of cold salted water. Bring to a boil over high heat, reduce to low, and simmer until potatoes are cooked through, but not falling apart, about 7 minutes. Drain well and add to the bowl with the vinaigrette. Toss well; cool to room temperature.
Add zucchini, fennel, radishes, scallions, parsley, and capers to bowl; toss well and serve immediately.