Preheat oven to 350 degrees. Grease a medium size baking dish.
Peel potatoes and thinly slice into disks. Fill a stockpot with water and bring to a boil.
Add the potato disks to stockpot, bring to a boil and simmer for 5 minutes. Strain potatoes.
While potatoes are simmering, heat oil in a skillet, add diced bacon and cook until golden. Add the diced onions to the bacon and cook until onions are soft. Set aside.
In the stockpot used to cook the potatoes, melt butter, and whisk in flour. Cook for 1 minute, then whisk in stock. Bring to the boil, then remove from heat. Set aside.
In a baking dish, add a layer of par cooked potatoes and top with half of the onion and bacon mixture. Season with sea salt and fresh cracked black pepper.
Add the rest of the potatoes and then pour the prepared roux over the potatoes season with sea salt and fresh cracked black pepper. Top with the rest of the onion and bacon mixture.
At this point, the baking dish can be cooled to room temperature and then stored in the refrigerator for up to two days. *If making head of time, bring baking dish to room temperature before baking in the oven. Baking time may need to be increased to ensure potatoes are heated through completely.
If baking immediately, cover baking dish with parchment paper and bake for 30 minutes. If using cheese, remove parchment paper and sprinkle top with cheese. Return to the oven for another 10 minutes or until cheese is melted.