Heat a saucepan over medium heat. Add oil and let it heat up a bit. Add onion and garlic and cook for 2 mins. Add the flour and herbs, 1/4 tsp black pepper and salt and mix in. Cook for half a minute, add in the lemon zest, miso and mustard and mix in and cook for another few seconds.
Add in half cup of the milk and mix in, add in the another half cup and mix in so that there are no lumps. Add in the rest of the non dairy milk and mix in and bring to a boil. Cook for 3 mins then take off heat. Carefully taste and adjust salt and flavor. This Bechamel sauce will be not be very thick but will continue to thicken as it cools.
Peel and slice the potatoes into an 1/8th inch thick slices. I do this while or after cooking the cream sauce, so the potatoes don't brown. (If slicing before hand, then store in a bowl of cold water until ready to bake. Then drain, lightly pat with a paper towel and move to the next step.)
Grease a baking dish or cast iron pan and arrange the slices in an overlapping fashion. You can also place a layer of non-overlapping potatoes, then place over this layer and repeat until finished. You want less than 3 layers for even cooking and crispy topping.
Pour all of the sauce over the potatoes. It will look like a lot.
Bake for 1 hour at 350 deg F (176 C) . Check in the center with a toothpick, there should not be any resistance. Continue to bake longer if needed. Broil for a minute to brown the top if needed.
Garnish with the remaining black pepper and some fresh herbs such as chives. If you are like me you would also sprinkle some red pepper flakes and maldon sea salt and serve.
Gluten-free: Use 1.5 tbsp rice flour and 1.5 tbsp starch such as tapioca starch and a thicker non dairy milk such as soy milk or cashew milk or thick oat milkOil-free: Dry toast the flour on a skillet over medium heat for 4 to 5 mins, stirring occasionally and set aside. Saute the onion and garlic in 2 tbsp broth in step 1 until translucent and proceed. Add the flour after mixing in the herbs, miso and mustard. Add half cup non dairy milk and proceed with the rest of the recipe(step 2 onwards)For a Cheesy Gratin: Top the sauced Potatoes with vegan cheese shreds or cheese shreds + breadcrumbs + herbs and bake. Variations: Recipe is easily doubled. Or add other layers in between. Check with a toothpick in the center and continue baking as needed. Add some nutritional yeast to the sauce if you like for extra cheesy flavor. Nutrition is for 1 serve