Preheat oven to 375°. In a small bowl, whisk together water, apple cider vinegar, salt and honey. Add apples, and submerge. Pickle apples for 20 minutes, or make ahead and store in the liquid in a jar and refrigerate.
Place salmon skin-side down in a baking dish, and remove bones if necessary. Sprinkle with salt and pepper.
Combine mustard and maple syrup in a small dish, and brush over salmon to coat.
Bake 15–20 minutes, until salmon flakes easily with a fork.
Meanwhile, remove and compost stalks from fennel, reserving fronds for garnish. Slice bulb in half, remove core, and thinly slice.
In a medium bowl combine fennel, arugula, feta, pumpkin seeds and currants, reserving some pumpkin seeds for garnish.
To make dressing, whisk together lemon juice, honey and salt in a small bowl. Drizzle in oil to emulsify. Toss salad with dressing.
Portion salmon into four pieces, gently removing from skin. Divide salad into four bowls and place on salmon. Top with a small mound of pickled apples, and garnish with reserved pumpkin seeds and chopped fennel fronds.