Season all the chicken pieces aggressively on all sides with an even coating of seasoning salt and black pepper. It should be a fairly heavy coating of both. Then season the pieces with the garlic powder and onion powder.

Place a large skillet over medium-high heat and warm the vegetable oil. When the oil is shimmering, add all the chicken pieces in a single layer; they should sizzle the moment they hit the pan. Let the chicken cook, keeping an eye on the heat and adjusting it if needed to make sure they don't burn, until fully browned on the underside, about 10 minutes. Once browned, flip them over, adding more oil as needed, and cook the second side until golden brown, another 6 minutes or so. The chicken will finish cooking in the gravy, so there's no need to worry about it being cooked through at this point. Transfer the chicken pieces to a large plate and set aside.

Turn the heat to medium-low; add the chopped onion, garlic, and celery; and cook, stirring, just until softened, about 2 minutes. Add the soup mix, Kitchen Bouquet, bouillon powder, and cayenne and stir to mix well. Then add the flour and cook, whisking constantly, until this roux is well mixed and nice and toasted, about 3 minutes. (If the roux is getting clumped inside of your whisk, add a little more oil to help it whisk more smoothly.) Add the water, about ½ cup at a time and whisking constantly, until you have a loose, fairly pourable gravy, about 5 minutes. Taste for seasoning and adjust with more bouillon powder if needed.

Return the chicken pieces in a single layer to the pan, submerging them in the gravy. Bring to a simmer, then cover, turn the heat to a low simmer, and cook until the chicken is completely tender, 45 minutes to 1 hour.

Serve immediately, with a scoop of mashed potatoes on the side.