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Scrub the sweet potatoes and prick all over with a fork. Place on a microwaveable plate and cook on High for 18-20 mins, or until tender. Split in half and place each one, cut-side up, on a serving plate.
Flake the drained tuna with a fork and divide between the sweet potatoes. Top with the red onion, and chilli if using, then squeeze over the lime juice. Serve with a dollop of yogurt and, if using, scatter over the coriander.