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1
Preheat the oven to 400 degrees F (200 degrees C).
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2
Heat 2 tablespoons butter and olive oil in a skillet over medium heat. Add carrots, celery, and onion. Add 1/2 teaspoon garlic powder; season with salt and pepper. Cook and stir until starting to soften, about 3 minutes. Add chicken; cook until heated through, about 3 minutes more. Add frozen peas; cook until turning bright green, about 1 minute. Stir and remove from heat.
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3
Melt 1/4 cup butter in a saucepan over medium-high heat until bubbling, about 2 minutes. Reduce heat to medium and add 1/4 cup plus 3 tablespoons flour. Whisk quickly to remove all lumps, about 1 minute. Cook, whisking slowly, until gravy is thickened, about 2 minutes more.
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4
Whisk chicken broth into the gravy thoroughly. Bring to a boil, stirring occasionally until thickened, about 5 minutes. Add the remaining garlic powder and onion powder. Season with salt and pepper. Add the vegetable-chicken mixture; stir and pour into a 9x13-inch glass baking dish.
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5
Combine 3 cups flour, 1 cup milk, and 1/2 cup plus 1 tablespoon mayonnaise in a bowl. Add remaining milk gradually until mixture is sticky; you may have leftover milk. Drop spoonfuls of dumpling batter to cover the top of the filled baking dish.
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6
Bake in the preheated oven until dumplings are light golden brown, about 25 minutes.
