First, make the marinade for the lamb. In a bowl, combine the vinegar, dried chilies, cinnamon stick, cloves, and cardamom. Allow to soak for 20 minutes to soften the chilies. Pour into a blender and add the cumin seeds, mustard seeds, black peppercorns, garlic, ginger, coconut milk, and salt. Puree for 2-3 minutes until completely smooth, adding extra vinegar to get a paste that's not too thick. Pour the marinade over the lamb, rub, and marinate in the refrigerator, covered, for up to 4 hours.
Make the raita: In a bowl, combine the yogurt, 2 tablespoons lemon juice, 1/2 teaspoon of salt and the cucumber. Stir thoroughly and refrigerate until ready for serving. The raita can be made up to 2 days in advance.
Make the u201cpickledu201d onions: In a bowl, combine the thinly sliced onions, 1/4 cup lemon juice, and 1/2 teaspoon salt. Cover and refrigerate for at least 30 minutes and up to 4 hours until the onions are slightly softened and lose some of their bite.
When the lamb has finished marinating, preheat the oven to 350 degrees F.
On the stove, heat a large deep, oven-proof pan over medium-high heat. Add the 2 tbsp of vegetable oil and heat until shimmering. Brush off any excess marinade from the lamb shanks, saving anything that remains.
Add the lamb shanks to the hot oil and brown for 4-5 minutes on either side until deeply caramelized. Remove to a plate.
To the remaining oil in the pot, add the diced onions and saute until golden brown, 4-5 minutes, adding more oil if needed.
Add the reserved marinade to the pot along with 2 cups of water. Add the coriander powder. Bring to a simmer, scraping the bottom of the pan to lift up all of the bits.
Add the reserved lamb shanks and any juices they have released. Add enough extra water to the pot so that it comes xbe of the way up the sides of the shanks.
Cover with a tight-fitting (oven-proof) lid or 2 layers of foil. Place on the middle rack in the oven and cook for 2 hours, turning the shanks over once in the middle. By the end, the meat should be falling off the bone.
Remove the shanks from the pot and, once cool enough to handle, shred the meat away from the bone. Discard the bones (or reserve for stock).
Place the pot with the braising liquid back on the stove and bring to a simmer. Reduce until it has the consistency of a thick stew. Add the shredded lamb meat to the liquid and add the garam masala powder and additional tablespoon of vinegar. Stir to combine and simmer for 3-4 minutes.
To serve, heat corn tortillas on a griddle until lightly charred. Fill each tortilla with the braised lamb and top with the raita, onions, and chopped cilantro.