This simple and elegant recipe is from chef Mark Kiffin (of Compound Restaurant, Santa Fe) and was published in Gourmet Magazine (Oct. 2003).
He recommendes using Carnaroli rice which you can find at Italian markets, but Arborio will do.
Wash leek well in a bowl of cold water, then lift out and drain. Blanch leek and cauliflower in a 4- to 5-quart pot of boiling salted water, uncovered, 1 minute. Drain in a sieve and transfer to a bowl of cold water to stop cooking. Drain well.
Bring broth and water to a boil in a 2- to 3-quart saucepan and keep broth at a bare simmer, covered.
Heat 2 tablespoons butter in a 4-quart heavy pot over moderately high heat until foam subsides, then sauté cauliflower and leek, stirring, 2 minutes. Add rice and cook, stirring, 1 minute. Add 1 cup simmering broth and cook at a strong simmer, stirring, until absorbed, about 2 minutes. Continue simmering and adding broth, about 1/2 cup at a time, stirring frequently and letting each addition be absorbed before adding the next, until rice is just tender and creamy looking, 20 to 25 minutes. (There may be leftover broth.)
Remove from heat and stir in cheese, remaining tablespoon butter, and salt and pepper to taste. Thin with a little remaining broth as desired and serve immediately.