At Veselka.
And here's one food blogger's description of the soup:
Impossibly clear, the deepest ruby red, redolent of spices and earthy mushrooms, Veselka’s Christmas Borscht appears for maybe 45 days a year sometime around Thanksgiving and disappears with the New Year. Born out of the meatless Christmas eve dinner customary in Eastern European countries, the broth is made of beets, laced with vinegar and savory spices and is served with a few tiny mushroom pierogis and a delicate dusting of dill. I could eat this soup every day for the rest of my life and when it’s around I do my best.
(And don't worry if you missed the first 18 parts of this series. They're available on demand.)