Place turkey breast in a medium saucepan, cover with stock, and add parsley, bay leaf, thyme, and peppercorns. Bring stock to a boil, reduce heat to low. Cover pot and simmer until turkey is just cooked through, about 20 minutes. Transfer turkey to a plate, remove herbs and peppercorns with a slotted spoon. When turkey is cool enough to handle, cut into 1/2-inch thick pieces and transfer to the buttered baking dish.
Add carrots and turnips to the broth and cook until just tender, about 10 minutes. Transfer carrots, turnips and thawed peas to the buttered baking dish. Strain broth into a bowl. Melt butter in the same pot over medium heat. Add leeks and shallots and cook until soft. Add flour and stir for 2 minutes. Stir in broth and white wine. Increase heat to high and bring to a boil, stirring constantly. Cook until reduced slightly. Add cream and cook until sauce thickens enough to coat a spoon. Season with salt and pepper and add the parsley and sage. Pour the sauce over the turkey and vegetables in the baking dish.
For the Crust: Place flour, herbs, and salt in a food processor and process until herbs are finely chopped. Add butter and shortening and process until mixture resembles coarse meal. Add a few tablespoons of the cold water at a time and pulse until the mixture just comes together. Gather dough into ball and chill for 30 minutes.
Preheat oven to 400 degrees F.
Roll dough into a 15 by 10 1/2-inch rectangle and place dough over onto filling. Trim overhang, tuck dough edge inside dish. Bake until crust is golden and gravy is bubbling, about 1 hour. Let stand 10 minutes before cutting.