At this time your spinach should be cooled and you need to squeeze as much of the water out of the spinach as possible. It can still be a little moist but not wet!
Mix your goat cheese, shredded mozzarella, and bleu cheese together with the spinach to make the stuffing
Now add in the 1/4 cup of bechamel you reserved (if it looks to dry add a little of the mornay into the mix, but the mixture should be soft and solid not runny.
Divide the stuffing between the flounder fillets and roll them, so that the flounder surrounds the stuffing.
Place the fillets in two individual baking dishes or one large enough to hold all four rolls without them moving around too much, make sure the fillets are seam side down so the flounder does not open up while baking.
Add a little water to the pan and place in a 350-degree oven for 15 minutes
Remove from oven and drain off the water, add your mornay sauce till it comes about 3/4 of the way up the stuffed flounder rolls and return to the oven to bake for an additional 10 minutes.
Remove from the oven and serve