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