In a large bowl, whisk together eggs, schmaltz and seltzer.
Then add matzo meal, onions, baking powder, salt and pepper. Mix just until combined. Do not overmix.
The mixture will be sticky, so refrigerate dough for 30 minutes or up to 4 hours so it is not as sticky.
Bring the stock to a boil, and then reduce to a simmer.
Shape the matzo mixture into 25 or so 1-inch balls, and place in the stock. Cover and cook for 30 minutes.
Then, add carrots, parsnips, celery and any other veggies you choose and chicken, and simmer covered for an additional 10 minutes until the balls are cooked and veggies are tender and balls are cooked through.
Add salt to taste and garnish with parsley or dill.