Heat the oven to 425°F and oil a rimmed sheet. Soak panko in milk for 3 minutes, then gently mix with turkey, egg, grated onion, meatball salt, allspice, nutmeg and pepper. Shape 24 equal meatballs and bake for 16 to 20 minutes until their centers reach 165°F.
Meanwhile, melt 1 tablespoon butter in a saucepan and cook the well-drained leek for 6 minutes until soft. Add broth, Dijon and soy sauce and bring to a simmer.
Pour hot broth mixture into a 4-quart slow cooker and add the hot meatballs. Cover and cook on HIGH for 2 hours, turning gently through the broth once after the first hour.
Whisk cornstarch with slurry water, stir into the sauce and cover on HIGH for 15 to 20 minutes until thickened. Turn the cooker off, wait for bubbling to settle and fold in cream; confirm the meatballs remain at least 165°F.
During the last 30 minutes, cover potato pieces with cold water by 1 inch, add potato-water salt and simmer for 12 to 15 minutes after boiling until tender. Drain and toss with remaining butter.
Divide potatoes among 6 plates and add 4 meatballs and gravy to each. Serve while the cream sauce is smooth and hot.