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, then squeeze over the lime juice. Top with a dollop of yogurt and scatter over the coriander, to serve.