In a shallow bowl, combine the all-purpose flour, salt, black pepper, onion powder, garlic powder, smoked paprika, and poultry seasoning.
Dredge each chicken thigh thoroughly in the seasoned flour mixture, shaking off any excess; reserve 3 tablespoons of the remaining seasoned flour for the gravy.
Heat the vegetable oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat.
Place the chicken thighs skin-side down in the skillet and sear for 6 to 8 minutes until the skin is golden and crispy, then flip and sear for 5 minutes before removing the chicken to a side plate.
Add the sliced onions to the same skillet and sauté for 5 to 7 minutes until softened and translucent, using a wooden spoon to scrape the browned bits (fond) from the bottom of the pan.
Stir in the minced garlic and cook for 30 seconds until fragrant.
Sprinkle the reserved 3 tablespoons of seasoned flour over the onions and cook, stirring constantly, for 2 minutes to form a roux.
Gradually whisk in the chicken broth and whole milk, stirring vigorously to ensure a smooth, lump-free gravy, and bring to a gentle simmer.
Return the seared chicken thighs and any accumulated juices to the skillet, nestling them into the gravy.
Reduce the heat to low, cover with a tight-fitting lid, and simmer for 30 to 35 minutes until the chicken is tender and reaches an internal temperature of 165°F.
Serve the chicken thighs and a generous ladle of gravy over portions of hot cooked white rice.