Preheat the oven to 350°F. In a shallow dish, whisk together the flour, onion powder, paprika, cayenne pepper, salt, and black pepper until well combined.

Pat the chicken dry with a paper towel, ensuring the surface is moisture-free for better browning. Dredge each piece in the seasoned flour mixture, coating both sides evenly. Shake off any excess flour, but reserve the remaining mixture for later use.

In a large ovenproof skillet, heat the oil over medium-high heat until shimmering. Add the chicken, skin-side down, and sear until a golden-brown crust forms, about 1 to 2 minutes per side. Once browned, transfer the chicken to a plate and set aside.

Reduce the heat to medium and add the sliced onions to the same skillet, using the remaining oil and browned bits for flavor. Season with salt and black pepper. Cook, stirring occasionally, until the onions soften and develop a deep golden hue, about 5 to 7 minutes. Stir in the minced garlic and sauté until fragrant, about 45 to 60 seconds. Sprinkle the reserved flour mixture over the onions and stir well to coat, allowing the flour to absorb any remaining oil.

Gradually pour in the warmed chicken stock while stirring constantly, ensuring a smooth consistency. Increase the heat to medium-high and bring the mixture to a gentle boil. Once bubbling, reduce the heat to maintain a steady simmer. Return the chicken and any accumulated juices to the skillet, nestling the pieces among the onions. Add the thyme and bay leaf for additional depth of flavor.

Carefully transfer the skillet to the preheated oven and let the chicken cook undisturbed for about 30 minutes. The dish is ready when the chicken’s juices run clear when pierced with a knife, and an instant-read thermometer registers 165°F at the thickest part of the meat.

Taste the sauce and adjust seasoning with additional salt and black pepper if needed. Remove and discard the thyme sprigs and bay leaf. Serve immediately, spooning the rich onion gravy over the tender chicken.