Taking flax works with every meal, it suppreses the insuline release (lower G.i.)
BUT not the post workout meal, this is basicially the only meal (well that and the meal right after it in which you would want a lesser one) you want a high G.I. meal so do not add flax.
Also remember nothing special about the evening which makes u fat, if you dont go over your total daily calories dont make a big deal of a bigger meal later on in the day.