Romano Risotto with Radishes

Name: Romano Risotto with Radishes
Ingredients: 6 cups reduced-sodium chicken broth (48 fl ounces); 2 cups hot water; 3/4 stick unsalted butter, divided; 1 medium onion, finely chopped (1 cup); 3 garlic cloves, finely chopped; 1 pound Arborio rice (2 1/2 cups); 2/3 cup dry white wine; 1/2 cup grated Pecorino Romano; 1 tablespoon white-wine vinegar; 1 tablespoon extra-virgin olive oil; 1 pound trimmed radishes, julienned; 1 tablespoon finely chopped chives; Accompaniment: extra-virgin olive oil for drizzling