Chicken
Melt butter in a saucepan over medium heat. Add onion and cook until translucent, about 5 minutes. Add flour, salt, pepper, celery seed, onion powder, and Italian seasoning and whisk until a paste forms, about 1 minute. Slowly whisk in chicken broth and then milk until incorporated; bring to a simmer and cook, whisking constantly, until sauce thickens, 3 to 5 minutes.
Remove thickened sauce from the heat; add vegetables (peas, carrots, corn, green beans) and cubed chicken/turkey. Stir until filling is well combined.
Make your chicken pot pie and bake in a 425F oven for 30-35 minutes. Cool for 10 minutes before cutting and serving.