Peel and quarter the potatoes, then place them in a large pot and cover with cold water and a teaspoon of salt.
Bring to a boil and cook for 15-20 minutes, or until fork tender.
While the potatoes cook, prepare the meatballs. In a large bowl, combine the ground turkey, breadcrumbs, cheese, egg, salt, pepper, and garlic powder, Mix until just combined, then roll into evenly sized meatballs. A 3 tbsp cookie scoop makes 12.
Heat a large skillet over medium-high heat and add some oil.
Sear the meatballs, turning occasionally, until lightly browned. Remove from the skillet and set aside.
In the same skillet, melt the butter. Stir in the cream, Worcestershire sauce, mustard, sage and garlic powder. Make a paste with the cornstarch and the water and add it to the pan.
Simmer, while stirring for 2-3 minutes until slightly thickened. Taste and adjust the seasonings as needed. If you prefer extra sauce, simply add more cream.
Return the meatballs to the pan and spoon the sauce over them. Simmer for another 2-3 minutes until heated through.
Drain the cooked potatoes and return them to the pot. Add the butter, milk, cream, salt, and pepper. Mash until smooth and creamy. Add a little more milk or cream if needed to reach your desired consistency.
Serve the turkey meatballs over a generous scoop of mashed potatoes and spoon the extra sauce over the top. Garnish with parsley and sage.