Ok, I think this is an American dish, I used to make it occasionally in a previous life and I've Dukanised it with a delicious (imho) result! Just had it for lunch and I'm oh so stuffed
500g lean beef mince
2 onions, halved and sliced
dollop concentrated tomato puree
Mix the mince with the egg and seasonings - I used salt, pepper, Italian herb blend, garlic powder and onion powder but use whatever you fancy. Shape into 6 patties and fry on both sides in a deep skillet. Don't worry about cooking them through as they'll finish off in the sauce. Remove from skillet and add onions and fry until limp and golden, then add some water (I didn't measure but not too much, maybe 300ml), the tomato puree and oxo cube and bring to the boil. Return the patties to skillet and cook through.
I didn't thicken the sauce because I wanted to stay away from any tolerateds but I understand you can thicken savoury sauces with a cooked pureed onion. Had this with cauliflower puree and it was devine! With enough left over to keep in fridge until day after tomorrow's pv day