• Set six 1-cup wide mouth jars or ramekins on a pan, and set aside.
• In a large saucepan, bring 6 cups water and 1 tablespoon kosher salt to a boil. Add sliced potatoes and cook until just tender, about 4 to 5 minutes. Drain and set aside.
• In a large cast iron skillet over medium heat, fry the chopped bacon until browned and just crisp, stirring regularly. Using a large slotted spoon, remove bacon to a plate lined with a paper towel. Pour off all but 2 tablespoons of the skillet's bacon grease, and then return skillet to burner over medium heat. Add 2 tablespoons of the butter. Then add onion and mushrooms, stirring to coat. Cook until they are softened, about 8 minutes, stirring regularly. Add garlic, thyme, and black pepper. Cook for another 2 minutes, stirring frequently, until garlic is fragrant. Remove skillet from heat and stir in cooked bacon. Discard paper towel from plate and remove skillet mixture to the same plate. Set aside.
• Set same skillet over medium heat once again (no need to clean it first). Melt the remaining 3 tablespoons of butter. While whisking constantly, sprinkle the flour over the melted butter to form a paste. Gradually whisk in cream, and then milk. Cook until mixture comes to a boil, whisking almost constantly. Then add Parmesan and soy sauce, and cook for 1 minute longer while whisking constantly. Add more freshly ground black pepper, if desired. Remove from heat.
• Place one tablespoon of cream sauce into the bottom of each jar. Layer in 3 of the larger rounds of potato into each jar, followed by another tablespoon of cream sauce. Add 1/3 cup of bacon and mushroom mixture to each jar, and press in flat with the back side of a spoon. Divide 4 ounces of the shredded cheese amongst each jar, and then add 3 more rounds of potato to each jar. Spoon two tablespoons of cream sauce over potato rounds in each jar. Top each jar with 1 tablespoon of the bacon and mushroom mixture, and then sprinkle the remaining 2 ounces of cheese over the top of each jar. Finish with some freshly ground black pepper.

To bake immediately:

• Preheat oven to 350°. With the jars on the pan, Loosely tent the jars with one large piece of foil, so the cheese doesn't burn. Don't press the foil down onto the jars. Place pan of jars in oven and bake for 25 minutes. Remove foil and then bake for another 10 minutes, or until cream sauce is bubbling and cheese is browned a bit. Remove from oven and serve hot.

To bake later the same day or the following day:

• Place gratins in a covered storage container and refrigerate until ready to bake. Take gratins out of refrigerator 30 minutes prior to baking. Preheat oven to 350°. Set jars on a pan and loosely tent the jars with one large piece of foil, so the cheese doesn't burn. Don't press the foil down onto the jars. Place pan of jars in oven and bake for 30 minutes. Remove foil and then bake for another 10 to 15 minutes, or until cream sauce is bubbling and cheese is browned a bit. Remove from oven and serve hot.