Position a rack in the center of an oven and heat the oven to 425°F.
In a small saucepan over high heat, reduce the pomegranate juice to about 1/4 cup (it should be syrupy), about 15 minutes.
Meanwhile, mash the goat cheese in a small bowl with a fork until smooth. Form the cheese into four 2-inch-diameter rounds. Put the hazelnuts on a small plate and press the goat cheese rounds into the nuts on all sides to coat. Transfer the cheese to a small baking sheet and bake until the nuts brown and the cheese softens, 8 to 10 minutes.
While the goat cheese bakes, melt 2 Tbs. of the butter in a 12-inch heavy-duty skillet over medium-high heat. Arrange the endives flat in the pan (they’ll fit snugly), sprinkle with 1/2 tsp. each salt and pepper, and cook, undisturbed, until browned, 2 to 3 minutes. Flip and cook until the other side starts to soften, 1 to 2 minutes. Transfer to a large plate.
Melt the remaining 1 Tbs. butter in the skillet, add the apples and shallot, sprinkle with 1/4 tsp. salt and cook, shaking the pan often, until the apples start to soften, 2 to 3 minutes.
Transfer the pomegranate juice to a medium bowl. Add the vinegar, mustard, and 1/2 tsp. each salt and pepper; whisk until combined. Gradually whisk in the oil and season with more salt and pepper to taste.
In a large bowl, toss the spinach and apples with half of the vinaigrette and season to taste with salt and pepper. Arrange the endives on 4 large serving plates, top with a mound of the spinach mixture, and then the goat cheese. Sprinkle with the pomegranate seeds (if using) and the chives and drizzle with the remaining vinaigrette. Serve.