Heat half the oil in a large frying pan; season and fry the chicken on a medium heat until golden brown. Transfer to a dish and set aside.
Add the remaining oil and heat over a medium heat; fry the onions and mushrooms for 6-8 mins until softened. Reduce heat to low, stir in the garlic and paprika and fry until fragrant. Pour in the stock, add Dijon mustard and Worcestershire sauce and stir with a whisk to incorporate. Add the chicken plus the resting juices and simmer on a low heat for 5-6 mins until slightly reduced and thickened.
Over a low heat, stir in the sour cream until well combined to create a creamy sauce. Simmer for 2-3 mins to heat through but do not allow to boil. Serve with rice or pasta.