Ravishing Radish Risotto

For risotto
6 cups Vegetable Broth
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
For radishes:
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

Make risotto: Bring broth and water to a simmer in a 3-to 4-quart saucepan. Meanwhile, heat 3 tablespoons butter in a 4-to 5-quart heavy pot over medium heat until foam subsides, then cook onion, stirring occasionally, until just softened, about 5 minutes. Add garlic and cook, stirring occasionally, until softened, 6 to 8 minutes. Stir in rice and cook, stirring, 1 minute. Add wine and cook, stirring, until absorbed, about 1 minute.
Stir 1 cup simmering broth into rice and cook, stirring constantly and keeping at a strong simmer, until absorbed. Continue cooking and adding broth, about 1 cup at a time, stirring frequently and letting each addition be absorbed before adding next, until rice is just tender and creamy-looking but still al dente, 18 to 22 minutes. Thin with some of remaining broth if necessary (you will have some left over). Remove from heat. Stir in cheese, 1 teaspoon salt, 1/2 teaspoon pepper, and remaining 3 tablespoon butter.
Prepare radishes: Whisk together vinegar, oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Toss radishes with dressing and chives. Serve risotto topped with radishes.