With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
On a work surface, using a chef’s knife or kitchen shears, remove the chicken’s backbone and push down to open flat. Cut the chicken in half.
In a bowl, combine the garlic, ginger, oil, salt, curry powder and paprika. Place the chicken on the prepared sheet, skin side up, and rub with the spice mixture. Season with pepper. Roast for 25 minutes.
Place the cauliflower florets in another bowl. Drizzle with oil and season with salt and pepper. Add the cauliflower to the baking sheet and toss with the pan juices. Continue roasting for 45 minutes or until a meat thermometer inserted into the chicken thigh, without touching the bone, reads 180°F (82°C) and the cauliflower is tender.
Scatter the onion rings and herbs over the chicken and cauliflower. Add dollops of the condiments and, if desired, sprinkle with a pinch of curry powder and paprika.