- In a sauté pan over medium heat, melt the butter. Add the onion and celery and partially cover the pan. Cook, stirring occasionally, until the vegetables are soft and translucent, 3 to 4 minutes. Add the sage and season with salt and pepper. Remove the vegetables from the heat and stir in the turkey. Let cool to room temperature.
- In a shallow bowl, combine 2 of the eggs and the milk and whisk to blend. Place the 1 cup flour and the bread crumbs in separate shallow bowls. Line a baking sheet with parchment paper.
- In another bowl, combine the mashed potatoes, the remaining egg and the 2 tablespoons flour and fold until blended. Have a bowl of cold water nearby. Get your hands wet but not dripping with water. Place about 2 tablespoons of the mashed potato mixture in one hand and flatten into a thick disk. Add about 2 teaspoons of the turkey mixture and form the mashed potatoes into a ball, sealing the turkey mixture in the center. Roll the ball in the flour, then in the egg mixture and finally in the bread crumbs. Set the croquette on the prepared baking sheet. Repeat with the remaining ingredients.
- In an electric deep fryer, heat the oil according to the manufacturer's instructions to 365°F. Alternatively, in a heavy deep-fry pan over medium-high heat, pour in oil to a depth of 2 to 3 inches and heat to 365°F on a deep-frying thermometer.
- Carefully place 3 or 4 croquettes in the hot oil (do not crowd the pan) and cook, turning as needed, until golden brown, 3 to 4 minutes. Repeat with the remaining croquettes. Serve hot with gravy. Makes 12 to 15 croquettes.