hare: hare, wild rabbit, with a pun on "whore."
lenten pie: Mercutio is thinking of what, in the U.S.A. now (C.E. 2015), is referred to as a "pot pie," a stew of meat, vegetables, and sauce in a pie crust. During Lent, such a pie should contain no meat, but Lent lasts at least forty days, and so someone might put a bit of hare in the lenten pie.
Lenten pie