To begin, brush the baking dish with butter. Set it aside.
Preheat your oven to 375°(F).
Sauté the beef bacon in a large skillet until it is lightly browned, but not crispy. Transfer the bacon to a plate lined with paper towel.
Add the butter and diced onion to the skillet, and cook over low heat and sauté gently until the onions are tender.
When the onions are tender, stir in the flour. Let it cook for a minute.
Whisk in the milk ,salt, pepper, and thyme. Cook this over medium heat, stirring continually until it comes to a boil and thickens. Remove it from the heat.
Spread a third of your potatoes in the bottom of your prepared casserole dish.
Sprinkle in a third of the beef bacon and then spoon a third of the sauce over the top. Repeat with the rest of the potatoes, bacon and sauce.
Cover this with foil (see note) and bake for about 65-75 minutes, or until the potatoes are just about done. Remove the foil and bake 15 more minutes.
Let it rest a couple minutes before serving.
Enjoy!
Notes
I do not like foil to touch my food, so I lay parchment paper over the dish, then cover it tightly with the foil.