1. Preheat oven to 425 degrees. Melt butter in a medium saucepan over medium heat. Add shallots, garlic, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Cook, stirring occasionally, until shallots are translucent, about 3 minutes. Stir in 1 cup stock. Whisk remaining 1/2 cup stock into the flour in a small bowl. Whisk into shallot mixture. Bring to a boil, whisking constantly. Reduce to a simmer. Cook 5 minutes, whisking occasionally.

2. Add carrots and celery. Bring to a boil. Stir in chicken. Return to a boil. Remove from heat. Stir in chopped parsley and tarragon. Let cool. Stir in peas. Transfer to a 2-quart (8 1/2-by-12-inch) baking dish.

3. Brush 1 sheet of phyllo with melted butter. Sprinkle with parsley leaves. Repeat with remaining phyllo, parsley, and butter; do not sprinkle parsley on final layer. (Keep unbuttered phyllo covered with plastic wrap and a damp kitchen towel.) Drape crust over cooled filling; tuck in edges. Cut four 4-inch slits in crust for vents. Sprinkle with sea salt, if desired.

4. Bake until crust is golden brown and juices are bubbling, about 30 minutes. Remove from oven, and recut vents. Serve immediately in shallow bowls.