For the mashed sweet potatoes: Scoop sweet potato flesh into a large bowl and mash with a fork or potato masher. Mix in the cream, butter, coriander, ginger and egg yolks. Season to taste with salt and pepper. Scrape into a pastry bag and set aside.
For the topping: Mix the cheeses, breadcrumbs and fresh thyme together in a bowl. Set aside.
For the shepherd's pie: Preheat oven to 375 degrees F.
Render down the bacon in a large saute pan or braiser over medium heat until crispy. Remove to a paper towel-lined plate, leaving the fat in the pan. Add the ground turkey to the pan, and cook, stirring often and breaking apart with a wooden spoon, until evenly browned, 5 to 7 minutes. Remove from pan and set aside. Add the onion, leek and sage, and cook until caramelized, another 7 to 10 minutes.
Stir in the pine nuts, flour, tomato paste, coriander and ginger and cook for 2 more minutes. Add the chicken stock, carrots and meat, then return to a simmer and cook until reduced and nappe (when it thickly coats the back of a spoon), another 10 to 15 minutes. Taste for seasoning.
Top with an even layer of the sweet potato mixture. Sprinkle evenly with the breadcrumb topping. Bake until golden brown, about 1 hour. Sprinkle with chopped parsley and serve.