Preheat oven to 400°. On a lightly floured surface, roll each pastry sheet into a 12x10-in. rectangle. Cut one sheet crosswise into six 2-in. strips; cut remaining sheet lengthwise into five 2-in. strips. On a baking sheet, closely weave strips to make a 12x10-in. lattice. Freeze while making filling., Toss chicken with 1/2 teaspoon each salt and pepper. In a large skillet, heat 1 tablespoon butter over medium-high heat; saute chicken until browned, 5-7 minutes. Remove from pan., In same skillet, heat remaining butter over medium-high heat; saute onion until tender, 5-7 minutes. Stir in garlic and herbs; cook 1 minute. Stir in flour until blended; cook and stir 1 minute. Gradually stir in broth and 1 cup cream. Bring to a boil, stirring constantly; cook and stir until thickened, about 2 minutes., Stir in vegetables, lemon juice, chicken and the remaining salt and pepper; return to a boil. Transfer to a greased 2-1/2 qt. oblong baking dish. Top with lattice, trimming to fit., Whisk together egg yolk and remaining cream; brush over pastry. Bake, uncovered, until bubbly and golden brown, 45-55 minutes. Cover loosely with foil if it starts getting too dark. Let stand 15 minutes before serving.