Preheat the oven to 350.
Season the skin-side of the thighs with kosher salt, some freshly ground pepper, and dried oregano. Don't be afraid of salt. Use about a teaspoon and a half for four thighs.
Heat an oven-safe skillet (nothing teflon coated, and nothing with a plastic handle) with about 3 Tbs of extra-virgin olive oil. Don't be afraid of evoo...it is your friend. Heat it on high heat...don't be afraid of high heat...once you get the hang of this, you won't burn anything.
While the skillet is heating, slice two cloves of garlic however you best like your garlic sliced, keeping in mind that the smaller the pieces, the more intense the garlic flavor.
Once the oil is hot, place the chicken skin-side down in the hot oil. (trick: if you push the chicken to the side just a leeettle bit the second you put it down in the oil, it won't stick to the pan). Let it sit, skin side down, in your pan for a few minutes until the skin part is nice and golden.
After a while, you'll get a feel for how long it takes to get your chicken the golden color you like it. If you're new to this, check it after five minutes. If the chicken still looks kind of pale, leave it sit for a while. It is scary to leave chicken for that long on high heat if you are not used to cooking like this; have confidence in yourself, grasshopper, pay attention and you will not let it burn.
While it's cooking like that, season the bottom, skinless side with the same amount of kosher salt, freshly ground black pepper, and oregano you used on the skin side. Once you're ready to turn your chicken over, add your sliced garlic, THEN turn over your chicken. Quickly squeeze half a lemon over the chicken. Then put the whole skillet into the oven, and there it will rest for about 25 min or so, roasting away and cooking through to the bone.
After about 20 minutes, open the oven door and put about a Tbs of feta cheese on top of the chicken thighs. A small slice is nice if you're using fresh feta, or pack together about a tablespoon into a ball-like shape with your fingers if you're using crumbles.
Use tongs to put the chicken thighs on top of the spinach and mushrooms.
Add some kalamata olives (cold) to the finished dish for extra flavor, or some cold feta crumbles.And that's how you cook chicken without drying it out, with a nicely flavored skin.