Preheat oven to 350 degrees F. For sauce, melt the 3 tablespoons butter over medium heat in a medium saucepan. Cook onion and garlic in hot butter for 4 minutes or until tender.
Stir in flour, salt and pepper. Cook and stir for 1 minute. Stir in milk. Cook and stir for 7 minutes or until thick and bubbly. Remove pan from the heat. Stir in cheese until melted. Stir in canned peas and ham.
Arrange half of the canned potato slices in a greased 2-quart baking dish. Pour half the sauce over the potatoes. Repeat layers. Combine panko and the 1 tablespoon melted butter in a small bowl. Sprinkle over mixture in dish.
Bake, uncovered, for 30 minutes or until bubbly and topping is golden brown. Let stand for 10 minutes before serving.