For The Main For The Mustard Dressing

1. Preheat your oven to 350°F/180°C. To make the courgette puree, slice one-third of your courgettes and place them on a baking sheet lined with baking paper, along with the onion and garlic. Drizzle with one tablespoon of olive oil, season with salt and pepper, and toss to coat. Bake for about 30 minutes, or until the vegetables are tender. Set aside to cool slightly, then drain off any excess liquid. Transfer the roasted vegetables to a food processor and blend until smooth. Set aside.
2. Using a mandoline slicer, thinly slice the remaining courgettes diagonally. Then use a knife to cut them into matchsticks and add them to a bowl. Repeat the process with the apple and celery, then add them to the bowl along with the chives, nuts, and lemon juice. Toss to combine and set aside.
3. To make the mustard dressing, whisk together all of the ingredients in a heatproof bowl set over a saucepan of simmering water. Whisk for 2-3 minutes, or until the dressing is warm and aerated.
4. Heat two tablespoons of oil in a large skillet over medium heat. In two batches, cook the slices of sourdough bread for 1-2 minutes per side, or until they are golden brown. Drain them on paper towels and set them aside.
5. Divide the toasted sourdough slices among plates. Spread a generous amount of the courgette puree over each slice. Toss the courgette matchsticks with one-fourth cup of the dressing, then spoon them over the puree. Pour the remaining warm dressing over the top and serve.