Heat a 10 inch cast iron skillet over medium heat and cook the diced bacon
until nicely browned and crispy. Remove the bacon to drain on paper
towel and save the rendered bacon fat to brush on the potatoes later (if you
like you can use butter instead or if you do not have enough bacon fat you
can make up the difference with butter or more olive oil).

With mandoline or a sharp knife, steady hand, and patience slice potatoes
crosswise into 1/8th inch thick slices. Transfer slices to a large mixing bowl.

Repeat the slicing process with the small onion and separate out the
concentric circles of the onion. Add the onion rings to the bowl with the
potato slices.

Mix the potato and onion slices with olive oil, salt, pepper, and crushed red
pepper flakes, if using, until well combined.

Arrange potato and onion slices vertically in your cast iron skillet. Once the
skillet if filled with potato and onion mixture, brush the top of the mixture
with rendered bacon fat or melted butter.

Bake 1 hour and 15 minutes. Once potatoes are beginning to brown, add
thyme (if using) and bake until potatoes are cooked through and the tops of
the potatoes are crisp and golden brown (about 15-20 more minutes).