In a food processor or hand grater, grate the potatoes, parsnips, and onions. Place in a strainer and drain as much liquid out as possible. Transfer to a large bowl and add eggs, flour, sage, salt and pepper. By hand or with spoon, mix until well combined. It should be a bit thick and sticky. If the mixture is too loose, add more flour.
In a large frying pan, cover the bottom with grape seed oil about 1/4 inch high on medium heat. In the palm of your hands, form small patties of mixture while removing as much moisture as possible and place in the oil. Place as many as the pan can hold at one time while leaving a little space to be able to turn the latkes. When bottoms are browned, flip to the other side and press down to flatten if need be. When both sides are browned, place on paper towels to absorb the remaining oil. Repeat process until the mixture is finished. Refill oil when needed.