2) Add the pistachios to a blender along with the garlic, half of the lemon juice, the hot sauce, if using, hot water, a sprinkle of salt and pepper and the vegetable oil. Blend until smooth. If it doesn't form a paste, add more water and more oil and blend until smooth. The consistency should be like a thin hummus; don't worry if you feel like you're adding a lot of oil. Taste and add a little extra-virgin olive oil and more lemon juice if you want more zing, which I recommend. It cuts the oils from the pistachios. Set aside once you're happy with it.
3) Wash the spinach and dry thoroughly, so the leaves can be coated with the dressing.
4) Toss the spinach, apple and celeriac in a large serving bowl and season with salt and pepper, to taste. Pour half of the dressing over the salad and toss again. If you prefer more dressing, add it all, which I recommend. Let it sit for 5 minutes, then garnish with pistachios and serve.