Preheat the oven to 350 F. Line a baking pan with foil and bring a large pot of water to boil.
Trim the cauliflower and then break it into fairly evenly sized florets.
When the water comes to a boil, add the cauliflower and blanch 1 minute, then drain. Let it air dry 5 minutes or so.
Use a mortar and pestle to grind the cumin seeds with the salt, then add the chili powder, turmeric and oil and mix it and mash it up well.
Put the cauliflower florets on the foil-lined pan and drizzle the spicy oil on it, turning to coat well.
Put the pan in the oven and bake about 30 minutes, stirring now and then so the cauliflower roasts evenly.
Put the cauliflower in a serving dish and squeeze the lemon juice on it, then serve.