For the chapati: In a bowl, mix together the flour, yogurt and some salt to form a dough. Divide the dough into four pieces. Roll out each piece very thin and heat on a cast-iron griddle or skillet until bubbles form. Flip and cook on the reverse side.
Brush the finished chapatis with grapeseed oil and sprinkle with sea salt, nigella and cilantro.
For the Penang curry paste: In a small skillet, toast the cardamom, coriander and nutmeg until fragrant, 10 to 15 seconds. Remove from the heat and set aside.
Remove the stems and seeds from the dried chiles. In a cast-iron skillet, toast the chiles, shallots and galangal until slightly charred, 2 to 3 minutes. Remove from the heat.
Pour 1/2 cup boiling water over the roasted chiles and let steep for 30 minutes, until softened. Drain, reserving the steeping liquid.
In a blender, combine the toasted spices, toasted peppers, shallots and galangal, and the garlic and turmeric root. With the blender running, pour in enough of the chile steeping liquid to blend into a paste. Transfer to a bowl.
In a baking dish or resealable plastic bag, marinate the steak in 1/4 cup of the curry paste for 20 minutes or up to 2 hours. Reserve the remaining curry paste for sauce.
Remove the steak from the marinade, place on a hot grill and cook for 3 minutes per side. Remove from the grill and let rest for 5 minutes. Slice into thin slices and set aside
For the curry sauce: Heat a wok or large nonstick pan over high heat until very hot. Add the coconut milk and cook until the oil separates from the milk, about 6 minutes. Stir in 1 tablespoon of the curry paste, the shredded coconut, lemongrass, kaffir lime, fish sauce, palm sugar, cilantro root and salt. Simmer for 5 minutes to blend the flavors.
Add the sliced skirt steak to the sauce and heat through.
Serve the steak with the chapatis, and garnish with Thai basil, sliced chiles and lime wedges.