Prepare the lotus root: Peel off the skin and trim off the ends. Slice each rhizome into 1-cm thick rounds.
Create a batter by whisking together rice flour and all-purpose flour with 1 cup of plant milk. If it's too thick, add 1-2 tablespoons of water. The consistency should be thick enough to coat the lotus root with little dripping.
To fry the lotus root, add enough oil to a large pan or wok and heat it to 350°F (175°C).
Dip lotus root in the batter. Carefully, lower the lotus root into the hot oil. Add as many coated lotus roots as the pan fits with a slight gap. Use a tong or chopstick to flip while cooking. Fry until both sides are golden brown, about 3-4 minutes on each side. (tip: let crust forms before flipping) Fry for a little longer if you want a deeper color.
To air-fry, preheat to 390F (200°C). Then, grease the inner rack with oil. Place battered lotus root in and spray the top with additional oil. Air-fry until golden brown, about 10-12 minutes (please take a peek once in a while to avoid over cook) with a flip in between. Repeat either method until all lotus root is fried.
Prepare the sauce. Whisk together marmite, water, soy sauce, vegan oyster sauce, maple syrup, and sugar in a bowl until sugar is dissolved
Heat a pan over medium heat. Slowly pour the sauce in. Stir with a spatula and cook until the sauce starts to thicken.
Add the lotus root and quickly toss to combine.
Serve warm with sesame seeds with a bowl of rice.