Spoon yoghurt into a muslin-lined sieve over a bowl, cover and refrigerate until whey drains and yoghurt thickens slightly (1-2 hours). Stir in garlic and season to taste.
Dry-roast spices in a small frying pan, stirring occasionally, until toasted (10-20 seconds). Set aside. Add sesame seeds and pepitas to pan and dry-roast, stirring occasionally, until golden and toasted (30-50 seconds). Leave to cool.
Serve yoghurt dip topped with hazelnuts, seeds, spices and mint and drizzled with oil.