Fry onion and garlic in olive oil until softened on medium heat. Add chicken, brown and then cover until meat is white.
Add butter to the chicken mixture. Once melted, stir in flour to create a paste. Allow flour mixture to cook for about 2 minutes, then add wine, stock, cream and milk. Stir till well combined, then add pepper and mushrooms. Season to taste.
Allow to simmer uncovered for about 5 to 10 minutes, until green pepper is cooked, stirring every now and again. If sauce isn't thick enough, add a little cornflour mixed with water, and cook until thick enough to your liking.