Peel and slice potatoes and set aside in a bowl of cold water.
Saute onion with 1 tablespoons butter until golden. Add garlic and continue to saute until garlic is lightly colored. Remove from heat and set aside.
In small sauce pan melt 2 tablespoons butter add 3 tablespoons flour. Stir constantly until mixture begins to bubble. Slowly add milk and stir constantly bringing sauce to a boil. Reduce heat and simmer for 2-4 minutes until sauce is thickened.
Drain potatoes. Spray or grease 2 qt casserole dish and add potatoes, sauteed onions/garlic, and pour sauce over potatoes. Salt and pepper to taste (you can tell by tasting the sauce). Mix well. Sprinkle bacon pieces and cheese crumbles then lightly hand mix throughout dish.
Optional: you can sprinkle parmesan cheese on top and/or 1/4 cup bread crumbs mixed with 2 tbsp melted butter for a crispier topping.
Bake at 400°F for 1 - 1 1/2 hours uncovered or until top is golden brown and potatoes are tender. Cover with foil for the first 45 - 60 minutes of the cooking time if you add the bread crumb mixture or else it will burn with this high temp/long cook time.