Pat the chicken thighs very dry with paper towels. This is the most important step for getting a good sear. Any moisture on the surface will steam the chicken instead of browning it. Season both sides evenly with salt, pepper, and garlic powder.

Heat the olive oil in a 12-inch cast iron skillet over medium-high heat until the oil shimmers and just starts to smoke lightly, about 2 minutes. Lay the chicken thighs smooth side down in the pan. Do not crowd them and do not touch them after they go in.

Cook the chicken for 6 minutes without moving it. You will know it is ready to flip when it releases from the pan on its own with gentle pressure from tongs. If it sticks, give it another minute. Flip and cook the second side for 4 to 5 minutes, until the internal temperature reads 165 degrees F on an instant-read thermometer. Transfer the chicken to a plate and tent loosely with foil to rest.

Reduce the heat to medium. Add the butter to the same skillet. Once it melts, add the sliced mushrooms in a single layer. Let them sit without stirring for 2 minutes so they brown on one side. Then stir and cook another 2 minutes until they are golden and most of their moisture has cooked off.

Add the shallots and garlic to the mushrooms. Cook for 2 minutes, stirring occasionally, until the shallots soften and the garlic is fragrant. You should start to see some nice brown bits on the bottom of the pan from the chicken, which is exactly what you want.

Pour in the chicken broth and Worcestershire sauce. Use a wooden spoon to scrape up all the browned bits from the bottom of the skillet. Those bits are pure flavor. Let the sauce simmer for 2 to 3 minutes until it reduces slightly and coats the back of a spoon. Taste and adjust salt if needed.

Return the chicken thighs to the skillet and spoon some sauce over the top. Let everything warm together for about 1 minute. Garnish with fresh parsley and serve immediately over egg noodles, rice, or mashed cauliflower.