Dice the chicken fillet. Cut the bell peppers into strips. Cut the spring onions into rings. Halve the mushrooms. Chop the garlic into fine slivers.
Heat the oil in a large pan over medium-high heat. Add the chicken and fry for 3-4 minutes, turning occasionally, until lightly browned. Season with salt and pepper, then remove to a plate.
In the same pan, fry the bell peppers, mushrooms, and garlic over medium heat for about 2 minutes until softened. Remove and add to the cooked chicken.
Pour the milk into the pan and stir in the sour cream. Coarsely grate the Gouda cheese. Melt half of the grated cheese in the sauce, stirring until smooth. Season with cayenne pepper, salt, and paprika to taste. Stir half of the sauce into the meat and vegetable mixture until well combined.
One at a time, lightly brown the tortillas in a dry pan over medium heat for about 1 minute per tortilla.
Grease a large baking dish. Spread the meat and vegetable filling on each tortilla, roll tightly, and place seam-side down in the baking dish. Pour the remaining sauce over the rolls and sprinkle with the remaining grated cheese.
Bake at 175°C (347°F) for 20 minutes until the cheese is melted and the rolls are heated through. Cut the parsley into strips and sprinkle over the rolls just before serving.
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