Line a large baking sheet with parchment paper and set aside.
Place white chocolate in a microwave-safe bowl. Microwave in 30-second intervals at 50% power, stirring thoroughly after each interval, until completely smooth and melted (about 2-3 minutes total).
Pour melted chocolate onto the prepared baking sheet. Use a spatula to spread it into a rectangle about 1/4-inch thick, leaving a 1-inch border around the edges.
Immediately sprinkle cranberries and pistachios evenly over the surface, pressing them gently into the chocolate with your hand or the back of a spoon so they adhere.
Tap the baking sheet on the counter 3-4 times to settle the toppings and remove air bubbles. Sprinkle with flaky sea salt if using.
Let the bark set at room temperature for 1-2 hours until completely firm, or refrigerate for 30 minutes if you're in a hurry.
Once set, lift the bark from the pan using the parchment paper. Break into 20-24 irregular pieces by hand, or score with a knife first for cleaner edges.
