Preheat oven to 375°F. Cut the cauliflower into thick slices. You should get 2 slices from each head. (The leftovers scraps can be prepared the same way.) Finely chop the pistachios—you can do this in a food processor or by hand. Cover a baking tray with parchment paper. Brush both sides of the cauliflower slices with oil and then sprinkle with pistachios, thyme, salt, pepper, and some chili. Bake until golden, about 15–18 minutes. Serve warm with a simple salad.