Step 1

Preheat oven to 375°. In a medium saucepan, melt butter. Add onions and cook until soft, occasionally stirring for about 4 minutes add garlic then sprinkle flour over the mixture and cook, constantly stirring, for 1 minute.

Step 2

While whisking, slowly pour in the milk. Add Dijon and Thyme and bring to a simmer and continue to whisk until thickened, about 3 minutes. Stir in ham and season with salt and pepper.

Step 3

Layer half the potatoes in the bottom of a 8 x 8 baking dish and season with salt and pepper. Spoon half the sauce over and sprinkle with half the cheese. Repeat with remaining potatoes, sauce, and cheese finishing with a layer of cheese.

Step 4

Sprinkle parmesan and chives on top and cover the dish with foil, and bake until potatoes are soft, about 40 minutes. Remove foil and bake until cheese is golden and bubbly, about 25 minutes more.