To make matzah balls, separate the egg whites from the egg yolks. In a large bowl, whisk together egg yolks, stock, matzah meal, onion, olive oil, salt and pepper and baking powder. Do not over mix, this leads to dense balls. Then a separate bowl, beat the egg whites with a hand or stand mixer until you have stiff peaks. Fold the egg whites into the matzah mixture until just combined. Refrigerate for 30 minutes to an hour.
When ready to make soup, heat a large pot of water to a simmer. Shape the matzah mixture into 12-15 1-inch balls. Place balls into water and simmer for 45 minutes or until matzah balls are cooked. Check doneness by cutting one ball in half. The color should be uniform all the way through.
In a large heavy-bottomed pot or Dutch oven melt butter over medium heat. Add onions and cook for 7-8 minutes until onion is cooked though and translucent. If onion starts to brown, turn heat down. Then add garlic and cook for one more minute.
Then add the potatoes, asparagus, spinach, arugula, parsley and broth and turn heat back up to medium. Simmer covered for about 10-15 minutes until asparagus and potatoes are tender. Don’t overcook or your vegetables will turn pea green.
Turn off heat, and blend soup in a blender or with an immersion blender. Add additional broth if your soup is very thick.
Season with lemon juice, cumin, salt and pepper to taste. Serve with matzah balls.