Combine the first four ingredients in a small bowl to make the Seasoning Mix.
Trim off any fatty pieces and tubular-shaped ducts from the liver. Sprinkle 1 teaspoon of the Seasoning Mix evenly on each liver and gently pat it in.
Combine 1 cup of the flour with 1 tablespoon plus 1 teaspoon of the Seasoning Mix and place in a shallow pan.
In a 12-inch cast-iron skillet, heat 2 tablespoons of the vegetable oil until very hot, about 4 minutes. Dredge the meat, 1 or 2 slices at a time, in the seasoned flour, gently shake off any excess, and transfer to the frying pan. Fry the liver, turning once, until it is a rich golden or light nut-brown, about 3 minutes. Remove the liver slices from the skillet as they are cooked and keep warm while you fry the remaining slices.
After all the liver is cooked, add the remaining 2 tablespoons vegetable oil and the remaining ¼ cup flour to the same skillet and cook, whisking constantly, until the roux is a rich red-brown color, about 8 minutes. Stir in the stock, the remaining Seasoning Mix, and the fire-roasted vegetables. Bring just to a boil, whisking constantly, then reduce the heat to medium and simmer, whisking every 4-5 minutes, until the sauce is thick and smooth, about 15 minutes.
To serve, drizzle each slice of liver with some of the sauce and pass the remainder separately.
Note: To fire-roast the vegetables: If you have a gas stove, simply place them right on the burner, in a high flame, and roast, turning with tongs, until the outer skin is charred all the way around. If your stove is electric, you can roast in a preheated 500°F oven. Plunge the roasted vegetables into ice water to stop the cooking, then rub off the black, charred skin under running water. It should slip right off, but if there are stubborn spots, just remove them with a sharp knife.