Put the potatoes into a saucepan of heavily salted, cold water.
Place the pan over a high heat, bring the water to the boil, then reduce the heat and leave at a gentle boil for 15-20 minutes, until the potatoes fall off the tip of a knife then drain in a colander.
Heat the frying pan and add the onion. Drizzle with oil, season with salt and pepper, then fry for 8-10 minutes, until golden and soft. Tip the onion out of the pan and reserve it for later.
Leave the drained potatoes to cool just enough to handle and slice the potatoes into even rounds, about 5mm thick.
Heat the pan over a medium heat. Add the butter and another drizzle of oil and, once the butter is bubbly, add the potato rounds. Depending on the size of your pan, you may have to cook the spuds in batches. You want to be able to lay each slice of potato flat on the bottom of the pan; the slices shouldn't stack on top of one another.
Once the potato slices are brown on one side (about 3 minutes), flip them over and do the same to the other side. Season each batch well with salt and pepper, then set the slices aside on a plate while you fry the remainder.
Repeat, adding oil as needed, until all the potatoes are golden on both sides.
Tip all the fried potatoes back into the pan together stil over a medium heat, and add the garlic. Cook for a couple of minutes. Then, add onion back to the pan as well as the paprika and steak seasoning and mix well. It should only take a couple of minutes for the onion to warm back up, then it's ready to serve.