Heat oven to 400°F and set a rack in the lower third.
Warm butter in a large skillet over medium heat. Soften onion for 4–5 minutes, then stir in garlic for 30 seconds.
Brown beef and pork in the skillet, breaking into small bits. Spoon off excess fat if needed.
Season with thyme, allspice, cinnamon, cloves, salt, pepper, and Worcestershire. Pour in broth and simmer until slightly reduced, about 8–10 minutes.
Fold in mashed potatoes and cook 2–3 minutes until thick and cohesive. Let the filling cool 10 minutes.
Fit one crust into a 9-inch pie plate. Spoon in the warm filling and smooth the top. Cover with the second crust, trim, seal, and crimp. Cut a few vents.
Brush the top crust with milk. Bake 35–40 minutes, until deep golden and bubbling at the edges.
Rest 10–15 minutes before slicing so the juices settle.