Heat a large cast iron skillet over medium-high heat and melt the butter. Add the onion, carrots, butternut squash, rosemary, and salt. Cook for 5 to 7 minutes.
Add flour and stir for 1 minute. Slowly add chicken stock and stir to combine. Add milk and simmer for 5 minutes to thicken. Stir in the peas and chicken.
Top mixture with pie crust, pressing the dough around the inside edges of the pan. Make a few slits on the top to allow steam to escape. Bake for 30 minutes or until crust is golden brown. Let sit for 10 minutes before serving.