Pat the chicken thighs completely dry with paper towels. Dry chicken browns properly. Season both sides with 3/4 teaspoon salt and 1/4 teaspoon black pepper.

Heat the olive oil in a 12-inch skillet over medium-high heat until it shimmers. Add the chicken thighs smooth-side down. Do not move them. Cook for 5 to 6 minutes until deeply golden and the edges look opaque about halfway up. Flip and cook another 4 to 5 minutes. The internal temperature should reach 165 degrees F. Transfer to a plate and tent loosely with foil.

Reduce the heat to medium. Add 1 tablespoon of butter to the same skillet. Once it melts, add the mushrooms in a single layer. Let them sit undisturbed for 3 minutes so they get some color. Then stir and cook another 3 to 4 minutes until softened and lightly browned. If mushrooms are crowded, cook them in two batches.

Push the mushrooms to the edge of the pan. Add the remaining tablespoon of butter to the center. Add the garlic and thyme and cook for 30 seconds, stirring constantly, until fragrant. Stir everything together.

Sprinkle the flour over the mushrooms and stir well to coat. Cook for 1 minute to get rid of the raw flour taste. The mixture will look thick and paste-like. That is what you want.

Pour in the chicken broth gradually while stirring, scraping up any browned bits from the bottom of the pan. Add the Worcestershire sauce if using. Stir until the gravy is smooth. Season with the remaining 1/4 teaspoon salt and pepper.

Return the chicken thighs and any resting juices to the skillet. Nestle them into the gravy. Reduce the heat to medium-low, cover, and simmer for 8 to 10 minutes until the chicken is heated through and the gravy has thickened enough to coat a spoon. Serve hot over egg noodles, mashed potatoes, or rice.