Heat oven to 400 degrees F. Butter bottom and sides of 9-inch pie pan.
In large saucepan, melt 2 tablespoons butter over medium heat. Add onion and
squash to pan. Cook, stirring occasionally, until vegetables begin to soften, about
10 minutes. Remove onion and squash from pan and set aside.
Add remaining 2 tablespoons butter to pan. When butter has melted, add flour,
whisking to combine. Cook for 1 minute.
Add chicken stock and heavy cream, whisking
to combine. Season with salt and pepper. Cook, whisking frequently, until sauce
thickens and coats the back of a spoon. Remove from heat.
Mix in chicken, kale and havarti, stirring well. Pour mixture into prepared pie pan.
Roll out puff pastry until one side is at least 10 inches long. Cut into eight
1-inch wide strips.
In small bowl, whisk egg with 1 tablespoon water or milk. Brush edges of pie
pan with egg wash. Arrange strips along top of pie pan, vertically and horizontally,
weaving to create a lattice effect. Press ends into edges of pie plate and trim
excess pastry. Brush tops of pastry strips with remaining egg wash.
Bake for 25 minutes, or until pastry top is golden brown and filling begins to bubble.