Peel and wash the potatoes. Dry well. Use a mandoline to slice them ⅛” thick.******
Arrange a rack in the center of the oven and preheat the oven to 400ºF/200ºC. Grease a 13x9” square baking dish with a bit of oil on the bottoms and up the sides.
Make a bouquet garni - tie your sage, thyme, rosemary and bay leaf in kitchen twine.
In a medium saucepan, add oat milk, bouquet garni, nutmeg, cayenne, peppercorns and salt. Bring to a simmer and then turn heat to low and gently simmer for 7 to 8 minutes. Once cooled, strain and save the infused milk.
In a deep 10 or 12-inch frying pan, heat 1 tablespoon of vegan butter or olive oil over medium-high heat. Once melted or hot, add the onion and season with salt and pepper. Stir fairly frequently until the onion is fully golden brown, about 15 minutes.
Add the remaining 4 tablespoons of butter or oil. Once melted, add the garlic and red pepper flakes and cook for 1-2 minutes, until the garlic is golden and aromatic, stirring frequently.
Add in the flour and whisk for 1 to 2 minutes to cook the flour into a paste. Gradually pour in the infused milk, whisking as you go to prevent clumping. Bring the sauce to a boil over medium-high heat. Then reduce the heat to medium and simmer, whisking often, for 3 to 5 minutes, until creamy and thick but pourable (it should coat the back of a spoon). Add in the nutritional yeast, mustard, and miso, and whisk until smooth. The sauce should be quite thick at this point. Take off the heat.
Add half of the potatoes to the bottom of the prepared pan in overlapping layers (it took us 3 layers to go through one half of the potatoes). Season the potatoes pretty well with salt and pepper. Top with half of the sauce and smooth it out evenly with a silicone spatula.
Add the remaining potatoes and season again with salt and pepper. Top with the rest of the sauce. Take a silicone spatula and and scrape any sauce stuck on the sides down into the potatoes (this helps prevent the sides from burning). Scatter the vegan parmesan evenly over the top.
Cover the pan with foil and bake for 1 hour, until the sauce is bubbly around the edges. Remove the foil and rotate the pan by 180º. Bake uncovered for 20 to 25 minutes, until a fork inserted all the way to the bottom gives no resistance and the top is a bit browned.Note: The top should be nicely browned by now, but if not, you can briefly stick it under the broiler for 2 minutes (watch closely!).
Sprinkle it with a bit of flaky salt and top with the chopped chives. Allow to cool for 15 minutes before slicing. It will thicken more as it rests.