Heat a knob of butter in a large pan and cook together the onion and leeks and cook until softened for 5 minutes. Add in the mushrooms and cook for 3 minutes. Remove from heat and transfer into a large bowl. Add in the breadcrumbs, cheese, chopped fresh herbs, cooked chopped chestnuts, zest of a lemon and 1 beaten egg. Season well with salt and pepper and put aside. Lay out a large piece baking paper covered with a sheet of foil. Generously butter the foil with garlic butter. Lay over the slices of bacon side by side. Next, lay out the turkey breast and beat slightly with a mallet to flatten out. Lay the turkey on top of the bacon and press down. Season the turkey well with salt and pepper then spoon over the stuffing mixture, leaving a 2cm area around the edges without any stuffing. Now tightly roll up, using the foil and baking paper to help you. Tighten up the edges, place the stuffed turkey on a baking tray and cook for 30 minutes. Remove the foil and continue to cook for another 15 minutes uncovered to brown. Remove from the oven and allow to sit for 10 minutes before slicing up into thick slices and serving with roasted potatoes and vegetables.