Peel 8 pounds of russet potatoes and keep under water until you are ready to use them. You can peel them the night before with no ill effect.
Preheat oven to 350º F. Peel and quarter 4 large onions. Place onions, 2 at a time in of the food processor or leave running until it’s a fine puree. Repeat with other onions. Using fine shredding disc (I did mine with the KitchenAid mixer attachment), process all the potatoes into a strainer sitting in a large mixing bowl. Press the potatoes with your hands to release any extra water; dump the potato water into a small bowl and reserve.
Beat 10 eggs in the same large mixing bowl. Add in potatoes, pureed onions, 2 TBL vegetable oil, 2 TBL kosher or sea salt, freshly ground black pepper (1+ TBL) and about a cup of matzo meal. Carefully pour off the water from the small bowl and recover the potato starch from the bottom: add that to the mixture as well. Mix thoroughly.
Place in large oiled baking dish (low, flat: I have a large oval dish that’s a bit bigger than 13×9 inches) and bake until golden on top. It will cook 60 minutes.