I have written above to chop the button mushrooms both halved and quartered. This is because they vary in size, in that if they are a little bigger I tend to quarter them and for the smaller ones just halve them. If they are really small I just throw them in whole. So adjust for what you have, if all are really small don't bother cutting them.
Cut chicken into approximately 1 inch pieces. Heat a small amount of olive oil in a pan and cook chicken in batches until browned all over. Remove and set to one side and wipe pan clean.
Pour about 2 tablespoons more of olive oil to pan and add the sliced leeks, cook for 2-3 Min's and then add the garlic, cook for a minute more (I do it this way so I do not burn the garlic.) Now add the mushrooms cook for a further 2-3 Min's.
I have a large, deep, fry pan that has a lid so I cook it all in the same pan, if you don't you will want to transfer the leek and mushroom mixture to a large saucepan before continuing.
Add the white wine, water, mustard, chicken stock, thyme and parsley, stir. Return chicken and cover, let it simmer for 20 Min's or until chicken is cooked through.
Boil or microwave carrots and sugar snap peas until tender.
Add cream to chicken, mix a little water with cornflour and add. Add the carrots and sugar snap and stir over heat until mixture boils and thickens.
Serve with either boiled new potatoes, mash or rice.
Also the beauty with a casserole is that there are no hard fast rules, so add whatever vegetables you want. Either your favourite or what you have on hand. This is also a great way to use up leftover roast chicken or turkey and it tastes great.