Preheat oven to 200°C | 400°F.
Season chicken with garlic powder, salt and pepper.
Heat cooking oil spray in a large, non stick and oven-proof skillet over medium-high heat until hot. Sear chicken thighs until golden and crispy on each side (about 3-4 minutes each side).
Transfer chicken to the oven and roast until completely cooked through, (about 25-30 minutes).
Once chicken is done, transfer to a warm plate and set aside. Drain some of the excess fat from the skillet, reserving 2 tablespoons for added flavour.
Return skillet to the stove over medium-high heat and sauté the garlic in the pan juices until fragrant (about 1 minute). Add the mushrooms, herbs and 2 teaspoons of parsley and fry until mushrooms begin to soften.
Reduce heat to low-medium heat, add the milk (or cream) and bring to a gentle simmer, stirring occasionally, and being careful not to boil. Add in the bullion powder and season with salt and pepper to your taste.
Pour the milk/cornstarch mixture to the centre of the pan, and continue to simmer while quickly stirring the mixture through until the sauce thickens.
Add 1/2 cup of parmesan cheese; allow sauce to simmer for a further minute until cheese melts through the sauce. Add the chicken back into the pan and allow to simmer for 1-2 minutes in the cream to take on the flavours. Taste test and add extra salt or pepper, if desired.
At this point, add in the optional add-ins, if desired. Allow spinach to wilt (if using).Sprinkle with the remaining 1/4 cup of parsley and 1/4 cup parmesan cheese.
Serve with steamed rice, over steamed vegetables or pasta.