Preheat oven to 400 degrees F.: You should feel the warmth of the kitchen change as the oven comes up to temperature, and a properly preheated oven ensures the roast phase crisps the skin rather than slowly cooking it to sogginess. Preheating also creates predictable timing for the internal temperature to reach the safe zone. A common slip is not preheating long enough, which can lengthen cook time and prevent browning. If your oven runs cool, use an oven thermometer to confirm accuracy.
In a large skillet (mine is 11" cast iron), heat up olive oil and butter over medium heat.: As the fat warms, you'll see a thin shimmer on the surface and smell a nutty butter aroma when it just starts to foam. This blend gives a higher smoke point and richer flavor than either fat alone. The hot fat is what creates an immediate sizzle when the chicken hits the pan, sealing juices and building Maillard flavor. Avoid overheating until the fats smoke, as that will introduce bitterness and ruin the buttery notes.
Season chicken thighs with salt and pepper and oregano.: The seasoning step should feel tactile as you rub the herbs and salt into the skin, and you should see the granules glint on the surface. Seasoning early helps draw moisture from the skin for better crisping and allows the oregano to adhere, releasing aroma as it heats. One mistake is under seasoning, which makes the final dish bland; taste the pan sauce at the end for balance and correct if needed.
Place chicken skin-side down in pan. Cook for 4 to 5 minutes.: When the thighs meet the hot fat you'll hear a satisfying sizzle and begin to smell roasting meat. After a few minutes the skin should turn a deep golden brown where it contacts the pan, and the thigh will release easily when properly seared. This initial sear builds a crispy exterior and concentrated flavor. If the skin sticks, it may not have rendered enough fat or the pan was too cool; give it another minute before trying to flip.
Flip chicken over, cook for 5 more minutes.: After flipping you'll notice the underside take on color and the pan accumulate browned bits and fat, which are flavor gold. This stage partially cooks the interior and continues to render fat from the skin. You want a warm, steady sizzle rather than an aggressive burn. If smoke rises excessively, lower the heat slightly and spoon some fat out if needed to avoid bitter flavors.
Add garlic, soy sauce and lemon juice to pan.: The moment you add the garlic the aroma will turn savory and pungent, while the soy sauce deepens the pan liquids and the lemon juice brightens them. These liquids will mingle with the browned bits to create a layered sauce. Stir gently so the garlic softens but does not burn, as burnt garlic becomes acrid. If your pan juices seem too salty after the soy sauce, a squeeze more lemon can balance it.
Place onion rings and lemon slices over chicken.: When you lay the red onion rings and lemon slices across the thighs they will catch the juices and roast into tender, aromatic companions. You'll notice the onion soften and the lemon rinds lightly caramelize, contributing sweet and floral notes. Distribute them evenly so each thigh gets some contact. Avoid piling too many slices in one spot, which can trap steam and prevent browning.
Place skillet in oven. Roast for 25 minutes OR until the internal temperature of each thigh is 165 to 175 degrees F. Spoon the pan juices over chicken half way through roasting.: In the oven the air moves heat evenly around the skillet, and you'll start to see bubbling juices and more concentrated color on the skin. Basting halfway through encourages even flavor distribution and keeps the surface glossy. Use an instant read thermometer to check doneness; probe near the bone for accuracy. A frequent error is relying only on time instead of temperature, which can yield undercooked meat or, conversely, dry thighs.
Remove from oven and let sit 10 minutes before serving. Drizzle pan sauce over chicken and serve.: Resting lets the juices redistribute, so when you cut into a thigh the meat remains juicy rather than losing liquid onto the plate. During this time the pan sauce thickens slightly and flavors meld, so the final drizzle coats the meat beautifully. Serve warm, and if the sauce seems thin after resting, return the skillet to the stovetop for a quick reduction. Cutting too soon causes the juices to run out, so patience pays off.