Heat oven to 325 F. Heat 2 tablespoons of the oil in a large ovenproof pot or Dutch oven over medium-high heat. Season the pork with the cumin, 11⁄2 teaspoons salt and 1⁄2 teaspoon black pepper. In batches, cook the pork until browned, turning occasionally, 6 to 8 minutes; transfer to a plate.
Add the flour and the remaining 2 tablespoons of oil to the pot and cook, stirring for 1 minute. Add the orange juice, tomatoes, garlic, jalapeños, and 4 cups water and bring to a boil. Return the pork to the pot, cover, transfer to oven, and cook until the pork is tender, 21⁄2 to 3 hours.
Meanwhile, in a small bowl, toss the onion with the vinegar, 1⁄2 cup water, and 1⁄2 teaspoon salt. Let sit, tossing occasionally, for at least 30 minutes (and up to 4 hours).
Skim off and discard any fat from the stew. Add the okra. Cover the pot, return it to the oven, and cook until the okra is tender, about 15 minutes. If the stew is too thin, place the pot on the stove and simmer, uncovered, until thickened, 5 to 10 minutes more.
Meanwhile, cook the grits according to the package directions. Serve with the stew and marinated onions.