Yeah, the haggis is an odd component to the dish, but I'm Scottish and it would be rude not to! Lol
As for the fricasse, I. Would start by heating a large casserole dish and add chicken stock, white wine, pearl onions, a few sprigs of tyme and bring to a simmer. Add the meat ( you can aslo use the carcass) and button mushrooms, then cover and simmer till the meat is cooked. Remove the meat (carcass) and take off the heat.
Now this is hard part, whisk egg yolks and cream together and gradually add to the stock to thicken the stock into a sauce. Adding to much will cause the egg to cook