Place the diced potato into a small pot. Pour in enough cold water to cover potatoes. Cover with lid and bring to boil. Reduce to simmer for 5 minutes or until they barely are fork-tender. Drain and set aside. Wipeout pot with a paper towel and set aside.
While potatoes simmer, render out bacon slices and brown. Remove to a paper towel and pour off excess grease. Saute onions in 1 tablespoon butter in the same pan until tender, 5-7 minutes. Remove from heat.
Place small pot you cooked potatoes in over medium heat. Melt in 2 tablespoons butter. Sprinkle in flour and whisk to create a paste. Cook 1 minute. Whisk in milk. Increase heat to medium-high and continue whisking until sauce thickens. Reduce heat and stir in cream, cream cheese, salt, pepper, nutmeg, and 1/2 cup grated cheese. Stir until cheeses melt and sauce is smooth.
Pour cooked potatoes, bacon, onions and sauce into a large bowl. Stir to coat. Pour entire contents of bowl into greased baking dish. Top with remaining 1/2 cup of cheese. Bake at 375 degrees for 20-30 minutes covered with foil. Place under broiler to brown the top if desired. Serve hot.