Preheat the oven to 400F and line two baking sheets with parchment. Toss the potatoes and apple with the vegetable oil and transfer to the baking sheets. Season with salt and freshly ground pepper. Roast for 35-40 minutes, or until the vegetables are browning and tender. Remove the vegetables from heat.
While the vegetables roast (or beforehand if it's easier), cook the rice according to package instructions.
Heat the olive oil in a large, deep skillet over medium heat. Add the onion, celery, and shallot. Cook, stirring often, for 5-8 minutes, or until the onion is soft and clear. Stir in the rosemary and thyme. Fold in the cooked rice, sweet potatoes, and apple. Add the 1/2 teaspoon salt and the vegetable broth. Heat everything through, stirring as you go.
Add the sherry vinegar and a big pinch of freshly ground black pepper. Taste and adjust the seasonings as needed. Transfer the stuffing to a serving dish and top with the pomegranate arils. Enjoy!