1. Preheat the oven to 180 °C fan.
2. Finely chop the pistachios in a food processor.
3. Blend the pistachios, breadcrumbs, butter, mustard and lemon zest into a smooth paste.
4. Put the feta into a small oven-proof dish and spread the pistachio mix over the top before baking for 10-12 minutes.
5. Cut the fennel and onions into very fine slices using a mandoline. Cut the tomatoes into fine slices.
6. Dress the vegetables with the olive oil, vinegar, salt, pepper and thyme.
7. Take the feta out of the oven and serve with the salad. Can also be served with a little toasted bread.