Preheat the broiler. Combine the chopped onion, ginger, Sriracha, rice vinegar, paprika, 1/2 teaspoon salt and 1/4 cup water in a blender; puree and set aside.
Season the pork chops with salt and pepper. Transfer to a baking sheet and broil until lightly browned and cooked through (do not turn), about 7 minutes. Tent with foil and set aside.
Meanwhile, heat the vegetable oil in a large skillet over medium-high heat. Add the cabbage, sliced onion and 1/2 teaspoon salt; cook, stirring, until softened and lightly browned, 2 to 4 minutes. Stir in the onion-ginger puree and cook until the cabbage is well coated and tender, about 2 minutes. Add the apple and watercress and toss to coat. Season with pepper.
Divide the pork among plates; drizzle with any juices from the baking sheet. Serve with the cabbage-apple mixture and sour cream.