1-First, preheat your oven to 350°F and grease a 9×13 or deep 9×9 baking dish with butter to prevent sticking.

2-Heat olive oil in a skillet over medium heat and cook the thinly sliced onions with salt and pepper until they turn golden and caramelized, then set them aside for later.

3-Next, clean the skillet and cook the bacon until it’s crisp, then drain the grease and wipe the pan clean to avoid any extra oil messing with the flavors.

4-Melt the unsalted butter in the skillet over medium heat, whisk in the all-purpose flour, and cook for about 2 minutes to form a base.

5-Gradually whisk in the whole milk and let it simmer for 2-3 minutes until the mixture thickens slightly, then remove it from the heat. This creates the creamy sauce that ties everything together.

6-Now, layer half of the sliced potatoes, caramelized onions, crumbled bacon, cheese, and sauce in your prepared baking dish, then repeat the layers for a hearty build-up.

7-Cover the dish with foil and bake for 1 hour, after which you remove the foil and bake uncovered for another 30 minutes until the potatoes are tender.

8-Let it cool for 10 minutes before serving to allow the flavors to settle.