Chicken Pot Pie Baked Potatoes

large russet potatoes

olive oil plus more for brushing

boneless, skinless chicken breast, cut into 1-inch pieces and lightly salted

unsalted butter

chopped onion

all-purpose flour

chicken stock or broth

whole milk

salt plus more for seasoning

frozen vegetable medley (carrots, peas, corn, and green beans)

panko breadcrumbs

dried parsley