Bring a large pan of salted water to the boil and cook the pasta according to pack instructions. Drain and set aside.
Meanwhile, heat the oil in a large frying pan over medium-high heat. Add the diced chicken and fry for 5-6 minutes until golden on all sides.
Add the onion and garlic. Cook for 3 minutes until the onion is soft.
Add the drained mushrooms and thyme. Stir and cook for 2 minutes.
Sprinkle the flour over the chicken and vegetables. Stir well and cook for 1 minute.
Gradually pour in the chicken stock, stirring constantly to avoid lumps. Add the milk and bring to a simmer.
Cook for 5 minutes until the sauce thickens. Season with salt and pepper.
Toss the cooked pasta through the sauce and serve.