American
Bring the water to a gentle simmer in a saucepan.
Add the frankfurters and heat them in the barely simmering water for 5-6 minutes; do not let the water boil hard, or the skins may split.
Alternatively, grill the frankfurters over medium heat for 5-7 minutes, turning often, until lightly charred.
Warm the buns in a 180C oven for 3-4 minutes, or toast the cut sides briefly on the grill.
Place a frankfurter in each bun.
Squeeze a line of mustard and ketchup along each frankfurter.
Add onion or relish if you like and serve at once.