Pat the pork dry, then season it with 1 1/2 teaspoons kosher salt and the black pepper. In a small bowl, combine the coriander, ground cumin, paprika, garam masala, turmeric, crushed fennel seeds, and cayenne if using.

Place the sweet potatoes in a medium saucepan, cover with cool water by 1 inch, and add 1/2 teaspoon kosher salt. Bring to a boil over high heat, then reduce to medium and simmer for 15 to 18 minutes, until very tender.

While the sweet potatoes cook, heat a wide Dutch oven or heavy deep skillet over medium-high heat. Add 2 tablespoons oil. Sear the pork in two batches for 3 to 4 minutes per batch, turning to brown two sides; transfer the browned pork to a plate.

Reduce the Dutch oven heat to medium and add the remaining 1 tablespoon oil. Add the onions and 1/2 teaspoon kosher salt. Cook for 8 to 10 minutes, stirring often and scraping up browned bits, until the onions are deeply softened and golden at the edges.

Stir the garlic and 1 tablespoon ginger into the onions and cook for 1 minute. Add the tomato paste and cook for 2 minutes, stirring constantly, until it darkens slightly. Sprinkle in the spice mixture and stir for 30 to 45 seconds, just until fragrant.

Pour in the apple cider vinegar and scrape the bottom of the pan. Add the chicken broth, crushed tomatoes, and brown sugar, then return the pork and any juices to the pot. Bring to a simmer, cover, and cook over medium-low heat for 30 minutes, stirring once or twice.

Uncover the pork and continue simmering for 10 to 12 minutes, until the sauce is glossy and the pork is tender enough to cut with a spoon. If the sauce gets too thick before the pork is tender, add a splash of broth or water.

Drain the sweet potatoes well. Return the empty saucepan to medium heat, add the butter, cumin seeds, and remaining 2 teaspoons ginger, and cook for 45 seconds until fragrant. Add the sweet potatoes and warmed milk, then mash until mostly smooth. Season with salt to taste, cover, and keep warm over very low heat.

Temper the yogurt by whisking it in a bowl with 1/3 cup of the hot masala sauce. Reduce the Dutch oven heat to low, stir the tempered yogurt back into the pork, and cook gently for 2 to 3 minutes without boiling.

Stir the spinach into the pork until just wilted, about 1 minute. Taste and adjust with salt or a small extra splash of vinegar for brightness.

Spoon the ginger-cumin sweet potato mash onto plates or shallow bowls. Top with the pork masala braise, then finish with cilantro, chopped cashews, and lime wedges.