Melt 50g of butter in another pan. Add the flour and whisk into a paste to create a roux. Cook the paste for about 2 minutes. Slowly add the milk and the stock in small doses, whisking constantly. You will end up with a milky sauce that is free from lumps. Reduce down until you have a thick, creamy sauce and season with salt and pepper to taste. Mix the chicken, shallots, garlic and the ham into the sauce.