The pie is a symbol of the crib, where the baby Jesus was laid. The mincemeat is the bitter/sweetness, representing His life. Sweetness because he came to save the world, bitterness because He had to die for our sins.
Centuries ago the mince pie would have been a large dish filled with various meats such as chicken, partridge, pigeon, hare, capon, pheasant, rabbits, ox or lamb tongue, livers of the animals, and...