Combine the dry mustard, vinegar, sugar and egg in the top of a double boiler and mix until smooth using a wooden spoon. Place over the heat and cook for four or five minutes until slightly thickened stirring with a wooden spoon.
Remove from the heat and stir in the miso. Season to taste with salt and pepper.
In a suitable bowl, combine the flour, baking powder and mint and mix well. Stir in the water and mix until smooth. Season to taste with salt and pepper. Chill.
In a deep fryer or deep pot (see tempura pan), heat the oil to 350 F.
Dip the beans into the batter and coat evenly. Remove from the batter and allow it to drip off slightly. Deep fry until golden brown.
Remove the fried beans to a baking pan covered with absorbent towels and sprinkle with salt and pepper. Serve with the sauce on the side for dipping.