Roast leek and onion, tossed in some of the olive oil, at 200 degrees C until tender, about 30 minutes.
Cut open squid tubes and score inside flesh with a diamond pattern then cut in half. Saute the squid, prawns and lemongrass in the rest of the olive oil, until tender. Mix with leek and onions. Place on plates.
Whisk together sweet chilli sauce and olive oil, then drizzle over the squid. Squeeze over the lemon juice then garnish with mint leaves.