Place chopped potatoes in large pot. Fill pot with water to just cover potatoes and bring to a rolling boil. Boil until potatoes are fork tender. Drain over a bowl to reserve water.
Place potatoes in a standing mixer (or mix by hand). Add about 1 cup potato water, ~2-3 Tbsp butter and sprinkle with salt. Mix on medium-high. Taste and add salt, additional butter, additional potato water etc. mix again and repeat until desired consistency and taste is reached. Set aside.
Preheat oven to 400 degrees F. Coat an 8x11 baking dish with nonstick spray and set aside. Spray a large skillet with nonstick spray and heat over medium-high heat. Add ground turkey and cook, breaking up along the way, until completely browned. Drain any fat and pour into prepared baking dish.
Add 1 T of oil to the same skillet. Add mushrooms, onion, thyme and 1/2 tsp of salt. Cook, stirring frequently, until the mushrooms are browned, about 5 minutes.
Add wine and cook until almost evaporated, 1-2 minutes. Meanwhile, whisk stock and flour in a bowl. Add to the skillet and cook, stirring until thickened. Stir in peas, carrots, remaining salt and black pepper. Transfer to baking dish with ground turkey, stirring well to combine. Spread mashed potatoes evenly over mixture. Bake 30-35 minutes until potatoes are browned and filling is bubbling.