Heat oven to 375°F and butter a 9x13-inch baking dish.
Boil noodles in salted water until barely al dente; drain well.
Melt 1 tbsp butter in a skillet; sauté onion 3–4 minutes, then garlic 30 seconds.
Whisk soup, milk, sour cream, mustard, salt, and pepper in a large bowl.
Fold in noodles, ham, peas, sautéed onion, and half the cheese.
Spread into the dish and sprinkle with remaining cheese.
Melt remaining 2 tbsp butter; toss with panko and paprika.
Scatter topping over casserole and bake 20–25 minutes until bubbling.
Rest 5 minutes, then garnish with parsley and serve.