You could mix some grated cheese in to the fromage frai and use that for your sauce for the pie?
Cook the fish first, put the cheesy fromage frais on top, then put your mash on top. Put under the grill to brown the mash slightly?
Depends if you're doing extra easy or red days. If it's for a red day, you could use carrot and swede mash on the top or slice up a potato (leaving a the skin on) and top with the potato slices.