Adjust an oven rack to the upper-middle position and another rack to the lower-middle position. Preheat the oven to 400°.
In a small bowl, whisk together the cumin, coriander, turmeric, salt, black pepper and cardamom.
In a large bowl, toss the cauliflower with the grapeseed oil and spices until well combined. Turn the mixture out onto two rimmed baking sheets and roast one pan on each rack for 30 minutes. Using a spatula, stir the cauliflower, then rotate the bottom pan to the top rack and the top pan to the bottom rack; return to the oven and continue to roast until tender and browned, 20 to 30 minutes longer. Transfer to a large bowl and serve warm or at room temperature.