"Hug in a mug" chicken soup

Name: "Hug in a mug" chicken soup
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: European, British
Ingredients: 2 sprigs fresh flat-leaf parsley, leaves only; 1 carrot, cut into pieces; 1 stalk celery, cut into pieces; 100 g fresh brown mushrooms, cut into halves; 150 g brown onion, cut into quarters; 50 g salted butter, cut into pieces; 500 g chicken breast fillets, cut into pieces (2 cm); 50 g plain flour; 2 pinches ground black pepper; 800 g water; 1 tbsp Chicken stock paste (see Tip); fresh crusty bread, to serve