Bring the potatoes to the boil in a pan of salted water. Reduce the heat until the water is simmering and simmer the potatoes for 8-10 minutes, or until just tender. Drain well and set aside.
Meanwhile, heat the oil in a deep-fat fryer to 190C. Alternatively, heat the oil in a deep, heavy-based saucepan, until a breadcrumb sizzles and turns golden-brown when dropped into it. (CAUTION: Hot oil can be dangerous. Do not leave unattended.)
Whisk the flour, sugar, salt, vinegar and half of the beer together in a large bowl to a thick, smooth batter. Continue to add the beer a little at a time until the batter has the consistency of double cream (you may not need to use all of the beer).
Separate the onion slices into rings and dip each one into the batter until completely coated. Carefully lower the onion rings, in batches, into the hot oil and fry for 3-4 minutes, or until golden-brown and cooked through. Remove from the hot oil and set aside to drain on kitchen paper. Repeat the process with the remaining onion rings.
Heat half of the butter in a frying pan over a medium heat and add the chopped onion. Fry for 2-3 minutes, or until softened.
Add the remaining butter to the pan, then add the garlic and drained potatoes and continue to fry for 2-3 minutes.
Add the chopped tomatoes and the corned beef and stir-fry for 1-2 minutes, or until the tomatoes have softened and the corned beef is heated through. Add the Worcestershire sauce and Tabasco and season, to taste, with salt and freshly ground black pepper.
To serve, spoon the corned beef hash onto serving plates. Pile the onion rings on top. Garnish with the sliced tomatoes.