Heat the oven to 200C/180C fan/gas 6. Bring a large pan of salted water to the boil and cook the cauliflower for 5 mins. Drain and spread out on a surface so any excess water evaporates.
Crush the fennel and coriander seeds using a pestle and mortar and mix with the paprika and a pinch of seasoning Put the cauliflower in a large bowl, drizzle with half the olive oil and sprinkle over the spice mix. Toss the florets to fully coat them.
Tip the florets onto a baking tray and space them apart. Put the red onions, peppers and courgettes on a separate baking tray, drizzle with the remaining oil, and cook both for 30-35 mins, turning halfway through so they brown all over and turn slightly crisp in places.
Mix the garlic with lemon juice and stir through the yogurt, adding a little extra water to loosen if needed. Stir the roasted onions, peppers and courgettes into the cooked quinoa along with the lemon zest and a pinch of salt.
Pile the quinoa salad onto a plate, then top with the cauliflower florets. Drizzle over the garlic yogurt.