Cut a sheet of kitchen parchment that will fit onto a baking sheet. Lightly flour the parchment. Unfold the thawed puff pastry onto it, then lightly flour the pastry. Using a rolling pin, roll the pastry into a 12- to 13-inch square. With a paring knife, cut three 2-inch long slits in the center of the pastry; these will serve as vents. Slide the parchment with pastry onto a baking sheet and refrigerate uncovered. Heat the oven to 375°F with a rack in the middle position.

In a 12-inch oven-safe skillet, preferably cast iron, over medium-high, heat the oil until shimmering. Add the onion and cook, stirring occasionally, until golden brown, about 5 minutes. Add the chicken and cook, breaking the meat into small bits, until the meat begins to brown, 5 to 7 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Stir in the carrots and potatoes. Add 3 cups water, followed by the curry brick. Bring to a simmer and cook, uncovered and stirring to ensure the curry dissolves, until the vegetables are tender and the mixture is thick enough that a wooden spoon drawn through the mixture leaves a slight trail, 15 to 18 minutes.

Off heat, stir in the peas, followed by the lemon juice. If the curry has continued to thicken, stir in just enough water to restore it to its consistency when it was removed from the heat. Taste and season with salt and pepper.

In a small bowl, beat the egg with 1 tablespoon water. Remove the pastry from the refrigerator. Peel the pastry off the parchment and center it on the skillet; using a silicone spatula, carefully tuck the edges down against the sides of the skillet. Brush the surface of the pastry with beaten egg (discard the remainder or reserve for another use), then sprinkle with the sesame seeds. Bake until the crust is golden brown, 30 to 35 minutes. Remove from the oven (the handle will be hot). Let stand for 10 minutes before serving.