Heat a large saucepan over high heat. Spray with olive oil. Cook tofu in 2 batches, turning, for 2-3 minutes or until golden. Transfer to a plate. Return pan to medium heat. Spray with a little more oil. Cook onion, stirring occasionally, for 5 minutes or until onion is soft. Add garlic, ginger, mustard seeds, cumin, turmeric and chilli flakes. Cook, stirring for 1-2 minutes or until aromatic. Add tomato paste. Cook, stirring, for 1 minute or until combined.
Add water and sweet potato. Bring to the boil. Reduce heat to low. Cook, covered, for 15minutes or until potato is tender. Add tofu, beans and broccoli. Simmer, covered, for a further 5 minutes or until vegetables are just tender. Stir through cashews, yoghurt and sugar. Top with extra yogurt, if using