Preheat oven to 400°F.
In a large skillet over medium-high heat, heat oil until very hot. Add mushrooms and cook until brown on all sides, about 6-8 minutes. Add in onion, carrot, celery and thyme, and season with salt and pepper. Cook until veggies are softened, about 3-4 minutes. Deglaze the pan with white wine and let it reduce by half. Remove from heat and let cool.
In a mixing bowl, combine turkey, panko, ground flax seeds, egg, 1/4 cup ketchup, nutmeg, salt, pepper, and the cooled mushroom mixture with your hands, but don’t overmix. Divide the meatloaf mixture into 6 equal parts and form each part into a mini meatloaf. Place the meatloaves on a parchment-lined sheet tray then brush remaining ketchup over the top of each loaf. Bake for 20-22 minutes, until meatloaf registers 160°F in the center.
While the meatloaves are baking, place sweet potatoes in cold water in a large pot. Turn the heat to high and bring to a boil. Cook sweet potatoes until fork tender, about 10 minutes. Drain and return to the pot. Add buttermilk, maple syrup, salt and pepper, and mash to desired consistency.
Serve 1 mini meatloaf per person with mashed sweet potatoes alongside.