Deluxe Turkey Potpie

In deep 12" skillet over medium-high heat, heat oil; add celery, carrots, onion and potatoes; cook 15 minutes, stirring occasionally until vegetables are lightly browned. Add garlic; cook 1 minute. Stir in chicken broth. Reduce heat to low; simmer, covered, 25 minutes until vegetables are tender. Meanwhile, heat oven to 425 degrees; have ready four 8" oval gratin dishes. If using homemade pastry, roll out on lightly floured surface to 1/8" thickness; or unfold all-ready piecrusts. Using bottom of one gratin dish as pattern, cut out 1 "lids"; cut out small hole in center of each and several slits as vents for steam. Arrange on ungreased large cookie sheet; brush with egg-yolk mixture (1 egg yolk and 1 tbsp water). Bake 10 to 12 minutes until lightly browned. In small saucepan over medium heat, melt butter; add flour; stir until smooth. Cook 2 minutes, stirring frequently; remove from heat. Stir mixture gradually into vegetables simmering in skillet; cook 2 to 3 minutes longer, stirring occasionally until thickened. Stir in peas and turkey; remove from heat. Spoon mixture into gratin dishes, dividing evenly. Place pastry lid on top of each; bake 5 to 10 minutes until filling is heated through.