From epicurious.com. Comments by mrm are in brackets .
Finely chop onion and trim stems close to tops of beets. Cut greens into 1/4-inch-wide slices and chop stems. Peel beets and cut into fine dice. In a small saucepan bring water to a simmer and keep at a bare simmer.
In a 3-quart heavy saucepan cook onion in butter over moderate heat until softened. Add beets and stems and cook, stirring occasionally, 5 minutes. Stir in rice and cook, stirring constantly, 1 minute. Stir in 1 cup simmering water and cook, stirring constantly and keeping at a strong simmer, until absorbed. Continue cooking at a strong simmer and adding water, about 1/2 cup at a time, stirring constantly and letting each addition be absorbed before adding next. After 10 minutes, stir in greens and continue cooking and adding water, about 1/2 cup at a time, in same manner until rice is tender and creamy-looking but still al dente, about 8 minutes more. [Note: I delayed adding greens until the very last minute.] (There may be water left over.) Remove pan from heat and stir in Parmesan.
Serve risotto topped with horseradish.
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