1. Thinly shave fennel bulbs into long ribbons using a mandoline or sharp knife.
2. Segment oranges by slicing between membranes to remove citrus segments.
3. Toast pistachios in a dry skillet over medium heat until fragrant (no oil needed).
4. Whisk together olive oil, lemon juice, salt, and black pepper for dressing.
5. Toss fennel, orange segments, pistachios, red onion, and parsley with dressing until evenly coated.