Place the chicken breasts into a bowl along with the mustard powder. Season well with salt and pepper. Use your hands to turn the chicken so that it is coated with the dry ingredients.
Chop the leek and peel the garlic.
Heat the oil in a frying pan over a high heat. Fry the chicken until it is very well coloured on both sides.
Add the leek, crush and add the garlic. Stir in the dijon mustard. Pour in the wine, turn down the heat and simmer for 15 minutes with the lid on, or cover your frying pan tightly with foil.
Remove the chicken to a chopping board and slice. Leave the sauce to bubble so that it thickens to your desired consistency.
To serve, spoon the sauce onto a plate and place the sliced chicken on top of the sauce.