For the spinach dumplings, defrost the frozen spinach. Finely chop the onion, chop the garlic clove and sauté with butter.
In a bowl, mix the breadcrumbs, spinach, flour and onion/garlic mixture. Form dumplings with wet hands.
In a wide pot, bring salted water to a boil and then add the dumplings. Immediately turn down heat and simmer gently for about 15 minutes.