Mix together all the ingredients of the spice rub. Sprinkle over both sides of chicken thighs and rub in to coat. Put chicken in gallon sized plastic bag and put in refrigerator for at least one hour.
Preheat oven to 400°F
After chicken has marinated for at least one hour, cut potatoes and onion. Place on 9x13 baking dish, drizzle with olive oil and kosher salt. Mix to coat.
Bake potatoes and onions for 30 mins at 400°F
Place large skillet over medium high heat. Drizzle olive oil in the bottom of the pan. Sear both sides of chicken for 2-3 minutes. Remove from pan and put chicken to the side.
Using the same skillet, add mushrooms and spinach. Add more olive oil if needed to keep things from sticking. Season with salt and pepper to taste. Cook until spinach is slightly wilted and mushrooms are soft. Remove from pan and place to the side.
Add butter and garlic to the pan over medium heat. Cook until butter is melted and garlic is fragrant. Be careful not to scorch the garlic.
Add flour and cook until combined.
Pour broth and milk into the pan. Cook on medium heat, stirring frequently. Continue to cook until sauce begins to thicken.
Add Parmesan cheese stir until melted into sauce.
Remove potatoes and onions from oven. Stir well to move any potatoes that may be sticking to the bottom of the pan.
Arrange chicken on top of the potatoes and onion.
Place spinach and mushrooms around the chicken.
Pour sauce over contents of pan.
Bake in 400°F oven for 20-30 minutes. Remove, serve, and enjoy.