Unfold the thawed puff pastry and seal seams using a rolling pin. Dock the sheet by piercing it several times with a fork. Cut into 12 (2-inch) rounds, place on a half sheet pan and refrigerate while preparing the rest of the casserole.
Heat oven to 400ºF.
Toss frozen vegetables with canola oil and spread evenly onto a sheet pan. Place into oven and cook until golden brown.
In a 12-inch straigh-sided saute pan set over medium heat, melt 1 tablespoon of butter and sweat the onion and celery until translucent, about 5 minutes.
While the vegetables are cooking, combine the broth and milk in a saucepan set over medium heat. Heat until just simmering.
Add the remaining 2 tablespoons butter to the celery mix and cook out the water. Add the flour and curry and cook for 1 to 2 minutes. Whisk in the hot milk mixture and cook until thickened, 3 to 4 minutes. Add the parsley, salt, and pepper. Add the browned vegetables and the chicken, and toss to combine. Pour into a 13-by-9-inch baking dish, or a large terra cotta pot base lined with foil, and top with the circles of puff pastry, spacing them so that they do not touch. Bake on the middle rack of the oven until puff pastry has browned and the mixture is hot and bubbly, about 25 minutes. Cool for 30 minutes before serving.