Boil the grated potatoes in water for 2-3 minutes, then drain and let cool.
In a bowl, mix the cooled potatoes, grated onion, minced garlic, flour, salt, and pepper.
Add the plant-based milk and apple cider vinegar to the potato mixture and mix well.
Heat a non-stick pan with vegan oil and form the potato mixture into 6-8 pancakes. Fry until golden brown and crispy.
For the mustard sauce, blend the Dijon mustard, whole-grain mustard, and vegan mayonnaise (if using) with a little water until smooth.
Serve the potato pancakes hot with the mustard sauce and a side of Rucola leaves.