Like corned beef in a Reuben sandwich

ON RYE

["Think of the bread that often pairs with pastrami or mustard in a classic deli sandwich, named after a grain."]

A Reuben sandwich is classically made with corned beef stacked between slices of rye bread, so "like corned beef in a Reuben" hints at the bread it’s served on. The answer "ON RYE" plays on the phrase "on rye" meaning both the bread type and the sandwich’s traditional presentation. It’s a straightforward reference to how the sandwich is typically ordered or described.