1. Cut chicken into thin 1 inch (about 2.5cm) strips.
2. In a medium skillet, heat 1 tablespoon olive oil. Fry chicken for about 5 minutes (until the meat is no longer red), then add onion and cook for another 2 minutes. Add the mushrooms, season with salt and pepper and cook until done (about 10 minutes).
3. To prepare creamy sauce, melt 2 tablespoons butter in a small saucepan, stir in flour. Then gradually add milk (a small amount at a time), stirring carefully to prevent lumps. Then add sour cream and season the sauce with salt and pepper.
4. Preheat oven to 350 degrees F (180 degrees C).
5. Lightly grease 4 small ramekins (one per person). Place about 2 tablespoons of the chicken and mushroom mixture in each ramekin. If you have some mixture left, divide it evenly between the ramekins.
6. Evenly pour the sauce over the chicken and mushrooms.
7. Put a thick layer of grated cheese on top
8. Bake uncovered for about 15 minutes, or until the top gained a nice golden brown color.
9. Sprinkle with parsley. Serve hot.