Shulchan Aruch (Y.D. 61 – 12) Matnos Kehuno (meats) should be roasted and eaten with mustard.
( should be eaten in a royal manner , like kings eat their food.)
One of the 24 Matnos Kehuno is, the sheep that is used to redeem a first born
donkey, and given to the Kohen. (pidyon peter chamor).