Fry the onion, tomato and chicken in a frying pan for 20-30 minutes using a dash of vegetable oil.
Make some mashed potato either traditionally or through a packet(enough for 1-2 people)-using le gusto its 0.5 of a sachet with 250ml water and 125ml milk.
Mix the Dijon Mustard, Mayonnaise and Lemon Juice together.
Put the Dijon mixture, Chicken mixture and mashed potato together in a casserole dish and mix together.
Sprinkle the breadcrumbs on top and cook for 30 minutes in an oven medium heat.