Preheat the oven to 200 degrees C (390 Fahrenheit)
Cut the chicken into big strips, place it in a bag, the add paprika, salt and pepper and close the bag. Shake it so that the chicken can be coated evenly.
Add 1 teaspoon of vegetable oil to a grill pan and cook the chicken until brown.
Bring 1 litre of salted water to the boil and boil the Brussels sprouts for 15 minutes or until they are tender.
Drain the water and place them in an ovenproof dish.
Add the cooked chicken and grate some parmesan and cheddar cheese on top.
Chop the walnuts and scatter them over.
In a pan, melt a knob of butter and add the plain flour, mixing well until you get a thick paste.
Add the milk and stir continuously until the sauce thickens.
Add the dijon mustard and pour the mixture over the sprouts, walnuts and chicken.
Grate some more cheese on top.
Season well with black pepper and bake in the oven for 15 minutes until the cheese has melted.