what is borsch? whats symbolically significant about this being zaroff's meal when we first learn about him?

 The standard definition for Broche is a soup made with beets and usually served with sour cream, associated with the cuisine of eastern and central Europe, especially Russia, Poland, and Ukraine. The soup reflects Zaroff's Russian (Cossack) ancestry, 

"the rich, red soup with sour cream so dear to Russian palates"