Grate onion in the food processor and remove to a large bowl. Grate zucchini in the processor and add to onion. Grate potatoes in the processor and add to onion and zucchini. The juice of onions prevents potatoes from turning black. (Alternatively, finely dice onions and grate zucchini and potatoes by hand with a box grater.)

Using your fingers, pick up small amounts of zucchini-potato mixture by the handful and squeeze out as much moisture as possible. Reserve all liquid and allow it to settle in a bowl for a few minutes. Put pressed mixture in a clean bowl. Carefully pour off watery part of reserved liquid but do not discard thick starchy sediment at the bottom of the bowl. Scrape that into potato mixture.

In a small bowl mix eggs, oil, salt and pepper and combine with potato mixture. Add matzo meal to batter, starting with 1/2 cup and adding more if necessary until potato mixture has enough body to form pancakes.

Heat a 1/2-inch depth of oil in a heavy skillet, preferably cast iron. Form potato mixture into pancakes of approximately 3 inches in diameter and drop into oil, taking care not to crowd pancakes. Fry, turning once, until golden on both sides for a total of about 6 minutes. Drain on clean paper bags. Keep pancakes warm while the rest are frying by placing cooked, drained pancakes in a preheated 200°F oven.