Melt butter in microwave oven. Mix in flour and then whisk in milk. Microwave 2 minute to slightly thicken. Stir in nutmeg, pepper flakes. Set aside.
Place the white and purple sliced potatoes into separate bowls.
Mix half of the red onion and garlic with each of the potatoes. Drizzle with olive oil just to coat and sprinkle with seasoned salt to taste and black pepper.
Toss to separate the slices and coat them with the oil and seasonings.
Layer half of the purple potatoes in the casserole dish then place 1/4 of the cheese over the layer of purple potatoes; layer 1/2 the white potatoes over the layer of cheese then top with 1/4 of the cheese. Repeat with remaining potatoes and cheese.
Pour the white sauce over the potatoes and using a knife help it into the layers.
Mix the panko and Parmesan cheese together and then sprinkle on top of the sauce.