Preheat your oven to 350 degrees fahrenheit. In a large ovenproof skillet, heat 2 tablespoons olive oil on medium-high heat. Add chicken thighs skin-side down and cook until skin is golden brown, about 5 minutes. Reserve the leftover marinade as you'll add that back in later.
Flip the chicken and cook another 5 minutes. Remove chicken thighs from skillet and set aside.
Use your tongs to scrape and remove any browned bits, and bunch up a couple of paper towels to soak up some fat from the pan, but not all. Reserve a little bit of grease to cook the onions.
Add the diced onions and stir for 1-2 minutes, or until they start to become translucent.
Add the chopped spinach, garlic, oregano, salt, pepper, and reserved marinade. Stir for another 30 seconds or until the spinach starts to wilt.
Add the rice to the skillet, and stir well to coat the rice with the oil.
Pour the chicken stock into the skillet and stir well. Bring this to a simmer on the stove.
Arrange chicken thighs on top of the rice, then cover the skillet and place in the preheated oven. Bake for 35 minutes. Remove the lid, return the skillet to the oven, and bake until chicken is cooked through and rice is tender, about 10 minutes more.
Let the chicken and rice rest for 5 to 10 minutes. The rice will look really dark as the spinach and onions rise to the surface. Just fluff the rice up with a fork to mix everything back together before serving.
Top with chopped parsley and grilled lemon slices or fresh lemon zest.