Combine the warm water and yeast and allow to sit for 2 minutes. Add the sugar and the oil. Now combine the yeast mixture with the flour in a large bowl or alternatively, in a food processor, and mix until you have a smooth dough.
Meanwhile, cut out 18 x 6cm (2½ in) squares of greaseproof paper.
Take the dough out of the bowl and knead it for a few minutes on a floured board, form it into a roll about 45cm (18in) long and about 5cm (2in) wide. Take a sharp knife and cut the roll into equal segments. There should be about 10 pieces. Roll each portion into a bun, then use a rolling pin to roll each bun out into an oval. Lightly grease each one, then fold them over a greased piece of baking paper. Put the buns on a greaseproof paper square. Do the same with the rest of the pieces of dough.
Place all the buns on a baking sheet, along with their greaseproof paper square. Cover the buns with greased clingfilm and let them rest for about 30 minutes in a warm place. The buns should have doubled in size.
Sprinkle sesame seeds on top of the buns.
Next set up a steamer over a wok or deep pan, and fill it with water. Bring the water to the boil over a high heat, then carefully lower the buns into the steamer (you may need to do this in batches). Turn the heat to low and steam over a high heat for 15 minutes.
Mix together the beef mince and a good grinding of pepper and shape into patties big enough to fit into fit into your buns.
Heat up a small amount of oil in a frying pan and add a few of the patties, add a good dollop of American mustard to the top side and a sprinkle of salt. Fry for 2 minutes and then flip over, cooking for a further 2 minutes on the other side. Remove from the frying pan and keep warm. When the buns are steamed, open the steamer and open the buns, adding a patty and a slice of cheese, before replacing the steamer lid and steaming for a further 2 minutes, until the cheese is melted.
Stir together the ketchup, mustard and chopped onion. Remove the buns from the steamer and spread a little of the relish on top of the cheese, adding a few pickles if you’d like.