Melt butter over medium-high heat in large pot. Add onion and cauliflower and cook until softened, about 8 minutes. Add garlic, curry powder, and red pepper (if using) and cook 30 seconds, until fragrant. Add flour and stir to coat vegetables. Slowly add broth, whisking constantly until sauce is smooth. Bring mixture to a boil, reduce heat, and simmer until thickened, 6–8 minutes. Stir in peas and chicken. Add parsley and season to taste with salt and pepper.
To freeze: Let filling cool completely and transfer to 1-gallon freezer bag or 2-quart freezer container. Freeze for up to 4 months.
To assemble/bake: The day before serving, remove filling and pie crust from freezer and thaw overnight in refrigerator.
Heat oven to 375°. Pour filling into a 2-quart baking dish. Unroll pie crust and roll it out to ⅛-inch-thick, if necessary. Place dough over baking dish, pressing any overhanging dough onto the sides of the dish. Cut several small vents in the surface of the dough. Place dish on a rimmed baking sheet and bake, uncovered, until crust is golden brown and filling is bubbling up, about 45–50 minutes. Let sit 15 minutes before serving.