2 leeks, white and pale green parts only, halved, rinsed well and sliced
3 carrots, peeled and diced
3 stalks celery, diced
3 cups baby spinach
1 pound cooked chicken breasts, chopped
1 cup frozen peas
¼ cup chopped tarragon
¼ cup chopped fresh parsley
2 Tablespoons snipped chives
12 sheets thawed phyllo dough
Preheat oven to 400°F.
Melt 6 tablespoons butter in a medium pot. Whisk in flour and cook, stirring, until pale golden, about 1 minute. Slowly add chicken stock, whisking until smooth, and cook until thickened. Add potatoes and bring to a boil. Reduce to a simmer, season with salt and pepper, and cook until potatoes are tender, about 15 minutes.
Meanwhile, melt another 2 tablespoons butter in a large skillet over medium heat. Add leeks, carrots and celery and season with salt and pepper. Cook, stirring, until vegetables have softened, about 10 minutes.
Add spinach and cook, stirring and tossing, until wilted, about 2 minutes. Stir in peas and chicken, season to taste, and cook until just heated through. Remove from heat.
When the potatoes are ready, stir in tarragon, parsley and chives. Season with salt and pepper if needed. Add vegetables and toss to coat in sauce. Transfer mixture to a large 4 quart baking dish. Set aside to cool slightly.
To assemble, melt remaining 8 tablespoons butter in a small saucepan. Gently unroll thawed phyllo and cover with a clean, damp cloth. Place a single sheet of phyllo on the work surface and brush with melted butter. Scrunch up the pastry and place it on the surface of the filling. Continue brushing pastry sheets, scrunching and placing the phyllo until the top of the pie is covered.
Bake for 30-35 minutes, or until the top is golden brown and the filling is bubbling. Serve hot.