For the dressing: In a small bowl, whisk together the pomegranate molasses, vinegar and mustard. Pouring in a steady stream, whisk in the olive oil. Season with 1/4 teaspoon salt. (The dressing can be made up to 3 days in advance.)
For the salad: Scatter the sweet potatoes and pearl onions on a rimmed baking sheet. Drizzle with the olive oil and season with 1 teaspoon salt. Roast until the potatoes are tender throughout and beginning to brown, about 35 minutes, tossing halfway through. Transfer to a large bowl to cool slightly.
Add the apples, baby kale and the dressing to the bowl, and season with 1/4 teaspoon salt. Using two wooden spoons, toss gently to combine and coat evenly in the dressing.
Sprinkle the salad with the pomegranate seeds before serving.