Preheat the oven to 400 degrees F (200 degrees C).
Place cauliflower florets in a large roasting pan. Pull apart onion quarters into separate layers and set aside.
Heat a small skillet over medium heat. Cook and stir coriander seeds and cumin seeds in the hot pan until slightly darkened, about 5 minutes. Remove from heat, crush coarsely with a mortar and pestle, and transfer to a medium bowl.
Add oil, vinegar, curry powder, garlic, paprika, chili powder, cumin, and salt to the crushed seeds; whisk to combine. Pour dressing over cauliflower; toss to coat. Spread in a single layer in the pan and sprinkle with pepper.
Roast in the preheated oven, stirring occasionally, about 20 minutes. Add onions and stir thoroughly. Continue to roast, stirring occasionally, until vegetables are tender, about 15 minutes more. Garnish with cilantro.