Mix together the minced beef, grated onion and beaten eggs.
Stir in Quaker Oats.
Add just enough matzo meal or breadcrumbs to absorb the egg.
Add salt and pepper to taste.
Mix well, then with wetted hands roll into small balls.
Heat the oil in a pan and briefly fry the meatballs to seal, turning once.
Transfer meatballs into a large casserole.
Lightly brown the chopped onion in the oil.
Add the chopped mushrooms and brown lightly.
Then add chopped tomatoes and the dissolved meat and mushroom stocks.
Bring to a boil, then lower heat and simmer for 5 minutes.
Pour stock over meatballs (stock should come to just half way up meatballs).
Cook in a moderate oven (Gas 4, Electric 160c) for about 45 minutes to 1 hour, turning meatballs once or twice during cooking.
Cooked meatballs can be frozen in sauce.