Preheat the oven to 200C/400F.
Place the cauliflower florets on a large baking sheet and drizzle with the oil. Sprinkle on the seasonings, toss together, and bake for 20 minutes, or until the cauliflower is golden and tender.
Meanwhile, prepare the couscous salad. In a large saucepan, heat the oil over medium heat. Add the onion and cook, stirring often, for 5 minutes or until the onion softens. Add the garlic, salt, black pepper, cumin, paprika and celery salt and stir together. Cook for an additional 2 minutes or until the spices release their fragrance.
Add the couscous and hot stock. Stir together and bring to a boil, the cover, turn off the heat, and let sit for 5 minutes.
After 5 minutes, remove the lid and fluff the couscous with a fork. Add the red and green bell peppers, tomatoes, spring onion and parsley and toss together.
Transfer to a large serving plate and top with the roasted cauliflower.
In a small bowl, stir together the mayonnaise and sriracha and drizzle on top of the cauliflower before serving.
