Preheat oven to 400 degrees.
Line a 13 x 9 baking dish with foil. Set aside.
Wash and dry the chicken thighs.
Add oil to a skillet to cover the bottom. Heat oil and add chicken skin side down. Sprinkle seasonings on the top of the chicken. Cook and brown for about 4-5 minutes and then flip. Season the top of the chicken and let the other side brown for 4-5 minutes. You are not cooking the chicken until done. You are just browning the outside.
While the chicken is browning, wash potatoes and dice into 2" pieces and place in the bottom of the prepared baking dish. Add spinach. Drizzle with vegetable oil. Toss to coat. Set aside.
In a small bowl, mix soup and milk. Stir until combined. Set aside.
Remove chicken from skillet and add to the top of potatoes in baking dish.
Pour the soup mixture over the top of the chicken and potatoes. Place butter slices on and around the chicken. This will melt and cook with the potatoes.
Cover with foil—Bake for one hour at 400 degrees.