Heat oil in a frying pan over medium heat. Add onion, garlic and celery. Sauté for 3 minutes or until beginning to soften. Add spices and cook, stirring until fragrant.
Add sweet potato and stock. Bring to the boil, then reduce heat, cover and gently simmer for 30 minutes or until sweet potato is soft.
Meanwhile, prepare seeded crackers. Place all ingredients in a bowl, stir to combine, then set aside for 15 minutes for the seeds to soak up the water.
Preheat oven to 180˚C and line a baking tray with baking paper.
Spread seed mixture over the baking tray and press out until 3mm thick. Back or 15-20 minutes or until golden brown. Allow to cool on tray, then break into pieces.
Remove soup from heat and stir through yoghurt. Use a stick blender to puree. Divide soup between 4 serving bowls, top with seeded crackers and serve.