In a medium bowl, whisk together the matzo meal, salt, baking powder, onion, and a bit of black pepper.
In a separate bowl, whisk together the egg yolks, club soda, and Schmaltz.
In a third bowl, using a hand mixer, beat the egg whites to stiff peaks, being careful not to overwhip.
Make a well in the dry mixture, then pour in the liquid mixture. Working quickly, use your hands to mix the batter until no dry patches or lumps remain.5 Still using your hands, vigorously fold in a third of the egg whites. Use a spatula to fold in another third, then the remainder.
Cover the mixture with plastic wrap so that it touches the sur- face of the batter. Refrigerate for at least 45 minutes, or preferably overnight.
Pour a bit of neutral oil into a bowl and rub a bit on your hands. Grab a piece of batter about 1 inch in diameter. Hold your hands together like you’re cupping water, cradling the batter around, letting gravity shape it into a ball. Do not apply pressure, or the balls will turn out dense! Repeat with the remaining batter to make 6 to 8 balls in total. Store any balls you’re not using imme- diately in an airtight container in the fridge for up to 1 week or in the freezer for up to 3 months.