Wash the potato, peel and cut in halves. Put in a deep pot, cover with salted water and bring to a boil. Reduce heat and simmer for another 20-25 minutes until tender.
Prepare filling, in a skillet over medium heat, add oil and sauce onion until fragrant about 2 minutes, then add garlic sit for another minute.
Add chicken cubes, stir for about 7-10 minutes until done, turn off heat, season with salt and pepper and stir in parsley.
When potatoes are done, drain well, mash with a fork or a potato masher. Add butter, milk, salt and pepper then mix all very well.
Let potato and chicken mixture cool so you can handle them.
Preheat oven to 350F.
Start preparing béchamel sauce, in another pot over medium-high heat, melt butter add oil then cook flour for a minutes until lightly yellow.
Carefully and gradually pour milk and keep whisking until no visible lumps are still in the mixture, season, reduce heat to medium and keep stirring until thickens.
Take a handful of the potato mixture and form into a ball.
Make an indentation in the middle and fill with a tablespoon or two of the chicken mixture, then roll the potato into a ball slightly bigger than golf ball.
Continue until all the potato and chicken are done.
Place the potato balls in an oven safe dish.
Crack and egg - if using- in the béchamel and whisk. Pour the béchamel sauce over the potato balls.
Sprinkle cheese on top.
Bake for 15-20 minutes or until cheese melts and casserole is bubbly.
Serve hot and Enjoy!!