Heat the oven to 350 degrees F. Combine the flour, onion powder, paprika and cayenne pepper in a shallow dish. Season with salt and black pepper and stir to combine.
Heat the oil in a large ovenproof skillet over medium-high heat until shimmering. Pat the chicken dry and place in the flour-spice mixture, turning to dust on both sides. Shake off the excess. Add the coated chicken to the hot oil and sear until brown, 1 to 2 minutes per side. Remove to a plate.
Add the onions to the oil remaining in the skillet and reduce the heat to medium. Season the onions with salt and black pepper. Cook, stirring occasionally, until the onions are golden brown, 5 to 7 minutes. Add the garlic and cook until fragrant, 45 to 60 seconds.
Tip the remaining flour mixture into the onions and stir to coat and combine. Stir in the warmed chicken stock and increase the heat to medium high. Bring to a boil. Reduce the heat until the mixture simmers. Add the reserved chicken with any accumulated juices, nestling it into the onions, and the thyme and bay leaf. Transfer the skillet to the oven and cook until the juices of the chicken run clear when pierced with a knife and the temperature reads 165 degrees F when measured with an instant-read thermometer, about 30 minutes. Taste and adjust the seasoning with salt and black pepper. Discard the thyme sprigs and bay leaf. Serve the chicken immediately.