Bring a large pan of salted water to the boil, add sprouts to the pan and parboil for 3 minutes. Drain in a colander and set aside.
Toast the cumin and coriander seeds in a small frying pan over a medium heat for 3 minutes and finely grind the toasted seeds with a pinch of sea salt with a pestle and mortar.
Tip most of the spice mix into a large roasting tray and toss in the sprouts.
Chop onions and fennel (or cabbage). Tip into the tray along with olive oil.
Spread everything in an even layer and cook for 20 minutes, until tender and starting to caramelise.
Meanwhile, return your frying pan to the heat and toast the sesame seeds and hazelnuts for 3 minutes.
Combine the yoghurt with the tahini. Peel and crush the garlic and stir through, then finely grate in half the lemon zest and squeeze in half the juice (save the rest for another day).
Taste the yoghurt and season well, then spread it evenly over the base of a large serving platter and sprinkle some sumac over the top.
Spoon the sprout mix on top of the yoghurt mixture, scraping up the lovely crispy bits in the pan. Sprinkle the ground nuts and seeds over the top.
Pick and finely chop the herb leaves, discarding the stalks. Scatter the leaves across the plate, then serve.