Tonight at Merkin Hall, the Mirror Visions Ensemble is presenting Concert à la carte. Its first half features food-themed works by American composers, ranging from art songs by Samuel Barber, Leonard Bernstein, William Bolcom, and Martin Hennessy to offerings from Broadway tunesmiths Stephen Schwartz and Cole Porter. But the second half of the concert is where the concept really kicks in. Mirror Visions has commissioned a new work from composer Richard Pearson Thomas. His cantata know thy farmer sets a number of texts drawn from the menus of Blue Hill at Stone Barns Restaurant. The evening also includes an introduction from Blue Hill’s executive chef