Preheat oven to 350F. Lightly grease a 9x13 or deep 9x9 baking dish with butter and set aside.

Add olive oil to a medium skillet and heat over medium heat. Add thinly sliced onion and salt and pepper to taste. Cook until onions become caramel in color. Remove from skillet and set aside.

Wipe skillet clean and cook bacon until crisp. Remove bacon from skillet, drain grease, and wipe clean again.

Melt butter over medium heat. Sprinkle flour on top of butter and whisk to form a roux. Cook for 2 minutes.

Slowly add the milk, whisking constantly. Bring roux to a simmer and let thicken slightly, about 2 to 3 minutes. Remove from heat and set aside.

Layer half of the potatoes, half of the caramelized onions, half the bacon, half the cheese, and half the sauce in the prepared baking dish. Repeat.

Cover baking dish with foil and bake for 1 hour. Remove foil and bake an additional 30 minutes or until potatoes are easily pierced with a fork. Remove from oven and let cool for 10 minutes before serving.