White Bean and Tuna Salad with Radicchio

Name: White Bean and Tuna Salad with Radicchio
Ingredients: 1 medium head of radicchio, cored, leaves coarsely torn; 6 tablespoons Parsley Vinaigrette, divided, plus more for drizzling; Kosher salt and freshly ground black pepper; 2 15-ounce cans cannellini (white kidney) beans, rinsed, drained; 2 celery stalks, peeled, sliced thinly on an extreme diagonal; 1 cup drained olive oil-packed tuna in a jar or good-quality canned tuna, broken into large pieces; Flat-leaf parsley sprigs (for garnish)