Button mushrooms join the chicken in the rich filling of this variation on our Classic Chicken Potpie.
Make the crust: In a food processor, pulse together flour, sugar, and salt. Add butter and pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Sprinkle with 3 tablespoons ice water. Pulse until dough is crumbly but holds together when squeezed (if necessary, add up to 2 tablespoons ice water); do not overmix. Form dough into a disk, wrap tightly in plastic, and refrigerate until firm, 1 hour or overnight (or freeze, up to 1 month).
Preheat oven to 375 degrees. Make the filling: In a large pot, melt butter over medium-high. Add onion and mushrooms and cook until softened, 8 minutes. Add garlic and cook until fragrant, 30 seconds. Add flour and stir to coat vegetables. Slowly add broth, whisking constantly until sauce is smooth. Bring to a boil, reduce heat, and simmer until thickened, 5 to 7 minutes. Stir in peas. Season with salt and pepper, then stir in chicken and marjoram. Pour filling into a 2-quart baking dish.
On a floured work surface, roll out dough to 1/8-inch thickness. Place dough over dish and fold overhang inward while pinching to crimp edge. Cut vents in dough. Place dish on a rimmed baking sheet and bake until crust is golden brown and filling is bubbling around edge, 45 to 50 minutes. Let cool 15 minutes before serving.
In the Fold Roll your dough a couple inches larger than your dish: Folding it up and over, pinching as you go, forms a beautiful decorative edge.
Switch It UpYou can bake these pies in any 2-quart baking dish. For personal pies, use six 10-ounce dishes.