Boil the diced potatoes in salted water for 8-10 minutes until tender, then drain.
Heat oil in a deep fryer or heavy saucepan to 190°C.
Whisk flour, sugar, salt, vinegar, and enough beer to create a smooth cream-like batter.
Coat onion rings in batter and deep-fry in batches for 3-4 minutes until golden.
Soften the chopped onion in 25g of butter in a frying pan over medium heat.
Add remaining butter, garlic, and cooked potatoes to the pan and fry for 3 minutes.
Stir in chopped tomatoes and corned beef for 2 minutes, then add sauces and seasoning.
Serve the hash on plates topped with onion rings and sliced tomato garnish.