Add more water if the vegetables seem crowded, then adjust the seasonings. Drain potatoes and mash them in a bowl. Add matzo meal and pepper. Knead dough until firm and smooth. Bring to boil.
Drop balls into boiling water.
Cook for 20 minutes in covered pot. Carefully remove from water and serve kneidlach in hot vegetable broth. Drain and mash potatoes. In a separate pan saute onion and mushrooms in oil and water over medium high heat for 3 minutes. Mix the mashed potatoes, sauteed mushrooms and onions, seasonings, and matzo meal together in large bowl.