The day before, put all the ingredients in a bread machine, and complete to the end of the 1st rising. Then divide it into 6 portions, round of each smoothly, and wrap each rather loosely in plasic wrap. Put the wrapped dough into a plastic bag and seal. Leave in the refrigerator overnight.
The next day, divide the ground meat into 100 g portions, and form them into round, flat patties. Season with salt and pepper and top with 1 tablespoon each of fried onions. Combine 2 tablespoons each of Japanese Worcestershire-style sauce (chuunou sauce) and ketchup. Use to top the patties.
Microwave the patties without covering for 10 to 15 minutes, until they are cooked through. They are delicious on their own. Try them in bentos.
Take the dough out of the refrigreator, round off each portion and press flat, and then leave to rise for about 20 minutes until doubled in volume (2nd rising). Time the buns so that they finish baking when the patties are cooked.
When the 2nd rising is finished, take the risen dough out of the oven (if you were using the 'bread-rising' setting). and start preheating the oven to 180°C. Mist the surface of the dough, and sprinkle with white sesame seeds. Bake in a preheated 180°C oven for 13 to 15 minutes.
Slice the buns in half, spread them with butter, and sandwich in lettuce leaves and the cooked patties to finish.
This is fried onion. You can find it at imported-food stores, or in the curry section of a supermarket. You can really make hamburger patties by putting this on ground meat and microwaving it! It's amazing. It costs about 400 yen for 200 g, and 800 yen for 300 g. It's really convenient to have on hand.