Season the boneless, skinless chicken breasts with salt and a generous amount of black pepper.
Preheat a frying pan over medium-high heat and add enough oil for cooking. Place the chicken breasts in the hot pan and cook for approximately 8–10 minutes, turning them halfway through so they cook evenly.
When the chicken is fully cooked, cut it into bite-sized pieces and transfer it to a mixing bowl.
Add the diced potatoes, condensed cream of mushroom soup, sour cream, and 1 cup of the shredded cheddar cheese to the bowl containing the cooked chicken.
Add a little more salt and pepper according to taste, then stir thoroughly until the chicken and potatoes are evenly coated with the creamy mixture.
Spoon the chicken and potato mixture into a casserole dish.
Spread the mixture into a reasonably even layer while maintaining a slightly rustic surface so the casserole cooks consistently throughout.
Transfer the casserole dish to an oven preheated to 350°F (177°C).
Bake for approximately 45 minutes, or until the diced potatoes have become tender and the surface has developed a light golden appearance.
Remove the casserole from the oven after the initial baking period and distribute the remaining 1 cup of shredded cheddar cheese evenly over the top.
If desired, scatter parsley flakes over the cheese for additional color and a subtle finishing touch.
Return the casserole to the oven for several more minutes, allowing the cheddar to melt completely and become hot and bubbly.
Take the finished casserole out of the oven once the cheese has melted.
Allow the dish to rest for a few minutes before serving so the casserole can settle and the flavors can come together.
Serve the chicken potato casserole warm.