Beef stroganoff

Name: Beef stroganoff
Prep time: 10 min
Cook time: 35 min
Total time: 45 min
Cuisine: European, Eastern European
Ingredients: 4 sprigs fresh flat-leaf parsley, leaves only; 1 brown onion (approx. 180 g), cut into halves; 30 g butter; 1 tbsp ground paprika; 1 pinch ground cayenne pepper, to taste; 700 g beef fillet, cut into strips (6 x 2 cm); 1 pinch salt, to taste; 200 g button mushrooms, cut into quarters; 50 g tomato paste; 100 g white wine; 1 tbsp Meat stock paste (see Tips); 1 tbsp cornflour; 1 tsp Dijon mustard; 120 g sour cream