Slice the onion into half-moons and the mushrooms 6 mm thick, and mince the garlic. Pat the thighs dry with paper towels and trim any large pieces of fat.
Mix the 30 g / 1/4 cup flour, 3/4 tsp salt, pepper, smoked paprika and garlic powder in a shallow dish. Press each thigh into the mixture until coated, then shake off the excess.
Heat the olive oil in a 30 cm / 12-inch cast-iron skillet over medium-high heat for 3 minutes, until it shimmers. Sear 4 thighs for 4 minutes per side without moving them, until deep golden brown. Move them to a plate and repeat with the other 4. Don't crowd the pan, because the thighs steam instead of browning. They won't be cooked through yet.
Lower the heat to medium. Add the butter, onion, mushrooms and 1/2 tsp salt to the same skillet. Cook for 8 to 9 minutes, stirring every minute or so, until the mushrooms are browned and the pan is mostly dry. Stir in the garlic and thyme for 30 seconds.
Sprinkle the 2 tbsp flour over the vegetables and stir for 1 minute, until no dry streaks remain. Pour in the broth a splash at a time, scraping up the browned bits from the bottom. Stir in the Worcestershire and cream, then bring to a simmer, about 3 minutes.
Nestle the thighs back into the gravy with any juices from the plate. Cover, lower the heat to medium-low, and simmer for 12 to 14 minutes, until the thickest thigh reads 175 F / 79 C.
Uncover and simmer 2 to 3 minutes more if the gravy doesn't yet coat the back of a spoon. Taste, adjust the salt and pepper, and sprinkle with parsley. Serve each plate with 2 thighs and a generous spoonful of gravy.