- Preheat oven to 400 degrees. In an 11-inch sauté pan or high-sided skillet, melt 2 tablespoons butter over medium-high heat. Add mushrooms to pan in a single layer (working in batches if necessary) and let sit undisturbed for 1 minute before stirring to allow to brown. Season with salt and pepper and cook 1 minute more, stirring occasionally. Remove mushrooms from pan and set aside.
- Add another 2 tablespoons butter to pan along with fennel and sauté for 10 minutes, stirring frequently, until fennel softens and begins to caramelize. Reduce heat to medium and add leeks; cook another 5 minutes until leeks are translucent. Season to taste. Return mushrooms to pan, add flour, and stir. Add chicken stock and bring to boil. Reduce heat and add cream. Add chicken to pan and stir to combine. Check seasoning.
- Working one sheet at a time, brush phyllo with melted butter and arrange loosely over top of mixture. Place in oven and reduce heat to 375 degrees. Bake for 45 minutes or until phyllo is nicely browned.