Preheat the oven to 425ºF (218ºC). Pat the chicken dry with a paper towel. Rub the oil on the skin-side of the chicken and season with salt and pepper.
Toss the mushrooms in the skillet and cook (undisturbed) for three minutes, over medium heat. Lower the heat to medium-low and add in the garlic and butter; toss to combine. Cook for 2-3 more minutes.
Add in the chicken broth and lemon juice, and stir to release any browned bits on the skillet. Place the chicken on top and transfer to the oven; cook for 17 to 20 minutes, until the chicken is cooked through.
Remove the chicken and set aside. Return the skillet to medium heat and whisk the remaining sauce until it starts to bubble and thicken, about 2 minutes. Add the spinach to the skillet and cook until wilted, about 1 to 2 minutes.
Place a cast-iron skillet over medium-low heat and once hot, add the chicken skin-side down. Cook for 5 minutes. Increase the heat to medium and continue cooking skin-side down for 8 minutes, until browned.
Turn the chicken over and cook for 2 more minutes, and transfer to a plate, skin-side up and set aside, leaving the oil from the chicken in the skillet.
Place the garlic butter chicken thighs on a plate, along with the spinach and mushroom mixture, and drizzle the sauce over everything. Enjoy!
