Preheat the oven to 400°F (200°C) and grease a medium dish (8x8 or 1.5 quart, or larger).
Heat a large skillet over medium heat. Melt the vegan butter in it and then add the onions. Cook them for 5 minutes or until they're caramelized. Add the garlic and let it sauté for about a minute until it becomes fragrant.
Sprinkle in the flour a little at a time. Stir as it coats the onions and garlic. Pour the plant milk in and stir as the sauce thickens. Stir in the vegetable broth. After a couple of minutes, it will return to a thick consistency.
Add the nutritional yeast, salt, pepper, paprika, and half a cup of vegan cheddar. Stir occasionally as it cooks for another 2 minutes.
Slice the potatoes at a thickness of 1/4 to 1/8 of an inch. Using a mandoline slicer will help speed this up and ensure uniformity in thickness. No need to peel the potatoes unless you want to.
Assemble the slices at an angle in the dish. You can also lay them flat if you prefer. Pour the cheese sauce over them and top them with the remaining cheddar.
Cover the dish with a fitted lid or foil and bake it for 40 minutes. Remove the lid and cook them for another 20 minutes. You’ll know they’re done when the cheese sauce is bubbling, and the potatoes are tender.
Let them cool for 5-10 minutes. Garnish them with fresh herbs such as thyme or parsley and enjoy!