COOK CHICKEN: Place a large saucepan over medium-high heat. Add 1 tablespoon unsalted butter. Once melted, add 1 pound diced chicken. Cook for 4 to 5 minutes, stirring occasionally, until the chicken is cooked through. Transfer the chicken to a plate, leaving the liquid in the pan.
MAKE ROUX: Add the remaining 1 tablespoon unsalted butter to the pan. Once melted, whisk in ¼ cup all-purpose flour. Cook for 1 minute, whisking constantly, until smooth and lightly golden.
BUILD BASE: Whisk in 2 cups chicken stock, 2 cups whole milk, 1 teaspoon poultry seasoning, 1 teaspoon onion powder, 1 teaspoon garlic powder, 1 teaspoon paprika, 1 teaspoon kosher salt, and ½ teaspoon fresh ground black pepper. Whisk until smooth and slightly thickened.
THICKEN SOUP: In a resealable jar or bowl, combine 1 cup of the hot soup mixture with the remaining ½ cup all-purpose flour. Shake or whisk until smooth. Slowly whisk this mixture back into the pot until fully incorporated.
SIMMER: Return the cooked chicken to the pot and add the frozen mixed vegetables. Reduce heat to medium. Cover and cook for 8 to 10 minutes, stirring occasionally, until the vegetables are tender and the soup is thick and creamy.
SERVE: Remove from heat. Garnish with chopped fresh parsley and serve hot.
Notes
Make-Ahead: This soup can be made up to 2 days ahead. Reheat gently before serving.Storage: Store leftovers in an airtight container in the refrigerator for up to 3 days.Reheating: Reheat over medium-low heat, stirring often. Add a splash of chicken stock or milk if the soup thickens too much.Freezing: Freezing is not recommended, as the milk-based soup can separate when thawed.Seasoning Tip: Taste before serving. If the soup tastes flat, add kosher salt ¼ teaspoon at a time, stirring and tasting between additions.