Preheat oven to 350°F. 

Shred cheese; set aside. In a medium saucepan, cover potatoes with water and add salt. 

Bring to a boil; cover and simmer 10 minutes, or until fork-tender. Drain well and set aside. 

Melt four tablespoons butter in a large skillet over medium-low heat. Add onion; sauté five minutes or until sweated. Add garlic; sauté two minutes or until fragrant. Add flour; cook two minutes, stirring constantly. Add milk and mustard powder; stir, bring to a simmer and cook, stirring constantly, until thickened. 

Add diced ham and green onions, reserving a few for optional garnish; cook two minutes or until ham is hot. Remove from heat and add two cups shredded cheese; stir until melted. Taste for salt; note that both diced ham and shredded cheese will affect saltiness during cooking. Add black pepper to taste.

Spoon potatoes evenly into a lightly greased two-quart baking dish. Pour ham and cheese mixture evenly over the potatoes; sprinkle top with remaining shredded cheese. Melt remaining two tablespoons of butter; toss with breadcrumbs. Evenly distribute over top of cheese. 

Bake at 350°F for 30 minutes or until browned and bubbly. Garnish with green onions if desired.