Applesauce and/or sour cream, for serving (optional)
Place the water and vitamin C tablet in a large bowl and stir until the vitamin C is dissolved; set aside.
Shred the potatoes through the shredding blade of a food processor. Remove the shredded potatoes and fit the food processor with the blade attachment. Working in batches, return the potatoes to the food processor and pulse until they are the size of rice grains. Add the potatoes to the vitamin C mixture and stir to combine.
Shred and pulse the onion in the food processor using the same method as with the potatoes. Add to the bowl of potato mixture along with the matzo meal, eggs, measured salt, and baking powder and stir until incorporated.
Pour 1/4 to 1/2 inch of the oil into a large frying pan and heat over medium-high heat. Once the oil is hot enough to fry (you can check by sticking a wooden utensil into the oil and seeing if bubbles form around the edges), use a large spoon to drop 3 to 4 mounds of the potato mixture (about 2 generous tablespoons each) into the hot oil. Do not crowd the pan. Flatten the latkes slightly with the back of the spoon.
Fry until golden brown, about 2 to 3 minutes per side. Remove the latkes to paper towels to drain and season with additional salt. Repeat with the remaining batter. When you reach the end of the batter, pat the mixture with a paper towel to remove any excess liquid before frying. Serve with applesauce and/or sour cream, if desired.