Trim the kidneys but leave a thin layer of fat around them.
Split them and trim away the tough center core.
Slice thin and sprinkle with salt and pepper.
Heat the oil and cook the kidneys quickly, shaking the skillet.
They should be browned but rare.
Pour into a colander and drain.
Wipe the skillet with a cloth or paper towels and add two tablespoons of the butter.
Add the shallot and cook briefly.
Add the wine and cook until it is almost totally reduced.
Add the brown sauce and bring to a boil.
Return the kidneys to the sauce and heat through.
When the sauce returns to the boil, remove it from the heat and stir in the remaining butter.
Add the mustard and, when it is thoroughly blended.