Using the coarse side of a cheese grater, grate your potatoes. Place the grated potatoes into a bowl of cold water and leave to sit for 20 minutes to allow the starch to release. Drain the potatoes and wrap into a clean tea towel. Squeeze all of the moisture out of them. You can also use just your hands for this step. Try to release as much moisture from the potatoes as possible and then place into a large mixing bowl.
Combine the flour, egg whites, garlic and onion powder and a generous amount of salt and pepper with the potatoes into the mixing bowl and mix well with your hands. The potato rosti mixture should be sticking quite nicely together however, you can add a little bit more flour if needed.
In a medium sized pan on a medium heat, add the olive oil so it is roughly 2cm deep in the pan. Bring the olive oil to heat.
Take a handful of potato rosti mixture and flatten well into the palm of your hands. Try to make sure the level of thickness is even throughout. Place 2 - 3 potato rosti into the pan and cook for 2 minutes on each side or until they are golden and crispy. Place the cooked potato rosti on a pre-lined plate with absorbent paper and repeat this with the remainder of the mixture.