Stir together yogurt, 1 teaspoon of the garlic, 1 teaspoon of the ginger, 1 teaspoon of the garam masala, 1 ½ teaspoons of the salt, and 1 teaspoon of the Kashmiri chili powder, in a large bowl until evenly combined. Add chicken and mix until evenly coated in yogurt mixture. Cover and refrigerate for at least 30 minutes or up to 24 hours (preferably overnight).

Preheat oven to 400°F. Melt butter in a 10-inch cast iron skillet over medium. Add onion and cook, stirring occasionally, until softened and starting to brown on edges, 8 to 10 minutes. Add cumin and remaining 3 teaspoons garlic, 3 teaspoons ginger, 2 teaspoons garam masala, and 1 teaspoon Kashmiri chili powder; cook, stirring often, until fragrant, 1 to 2 minutes. Add marinated chicken pieces (discard any yogurt mixture left in bowl), and cook, stirring occasionally, until opaque but not fully cooked through, 6 to 8 minutes. Stir in tomato puree and remaining 1 teaspoon salt; bring to a simmer over medium, 30 seconds to 1 minute. Simmer, stirring often, until slightly thickened, about 15 minutes. Stir in cream, then return to a simmer over medium, about 15 to 30 seconds; stir in peas and remove skillet from heat.

Roll pastry on a lightly floured surface into a 12-inch square. Working quickly, lay pastry over skillet; trim overhang to ¾- to 1-inch. Fold pastry under itself so rolled edge is flush with top edge of skillet; use the tines of a fork to crimp edge into side of skillet. (It’s OK if pastry doesn’t perfectly adhere.) Brush top of pastry with beaten egg and sprinkle with nigella seeds, if using. Cut 3 (1- to 2-inch) slits in top of pastry to vent.

Bake in preheated oven until pastry is golden brown and filling is bubbling, 25 to 30 minutes. Let stand 15 minutes before serving.