In a small sauce pan, saute the onions in olive oil over medium heat, for about 5 minutes.
Add the garlic and flour, and cook for another 3-4 minutes.
Now, add the turmeric, mustard, and veggie broth paste (or salt). Stir everything together nicely.
At this point, start adding the oat milk gradually, stirring the entire time. Bring it to a gentle simmer and then turn off the heat.
Using an immersion blender (or regular blender), process the sauce until smooth.
Peel the potatoes and cut them into ¼-inch thick rounds. Arrange them in a baking dish.
Sprinkle the potato slices with salt, and pour the sauce over the potatoes evenly. If desired, sprinkle some red pepper flakes on top.
Bake at 375°F for about 1 hour, until the top looks nicely browned and the potatoes are cooked through. Serve.