Place the chicken, breast-side down, on a clean work surface. Use kitchen scissors to cut along each side of the backbone to remove. Turn chicken over. Use the heel of your hand to press down on the breastbone to flatten. Season.
Combine the lemon rind, lemon juice, oregano, garlic, 1 tsp paprika and ¼ cup (60ml) olive oil in a large bowl. Season. Add the chicken and rub all over with the lemon mixture. Cover and set aside for 30 mins to develop the flavours.
Meanwhile, preheat a chargrill on high. Drizzle the corn with 1 1/2 tbs olive oil. Sprinkle with the remaining paprika. Season. Cook the corn on the grill, turning occasionally, for 12 mins or until tender and charred. Set aside to cool slightly. Use a small serrated knife to cut down the side of the cobs to release the kernels. Transfer the kernels to a large bowl.
Drizzle the spring onion with the remaining olive oil. Season. Cook on the grill, turning, for 2 mins or until tender and charred. Set aside to cool slightly. Coarsely chop and add to the corn in the bowl.
Preheat oven to 200°C. Line a large baking tray with baking paper. Place chicken, breast-side up, on the lined tray. Pour over any lemon mixture from the bowl. Roast for 50 mins or until the chicken is cooked through and juices run clear when the thickest part of the thigh is pierced with a skewer. Cover. Set aside for 10 mins to rest.
To make the buttermilk dressing, place buttermilk, yoghurt, oil, parsley, chive, lemon rind and lemon juice in a screw-top jar. Seal. Shake to combine.
Add the lettuce, parsley, mint and avocado to corn mixture. Drizzle with the dressing. Gently toss to combine. Season. Arrange on a large serving platter with the chicken.