Cut down each side of chicken backbone with kitchen scissors (discard bone or reserve for stock). Turn chicken over and press firmly on breastbone to open out and flatten. Score thickest part of breasts and thighs, place in a non-reactive dish.
Process garlic, thyme, peppercorns, cayenne, lemon rind and 2 tsp sea salt in a food processor until a coarse paste forms. Stir in lemon juice and olive oil, pour over chicken, turning to coat well and rubbing into flesh. Cover with plastic wrap and marinate in refrigerator for 2-3 hours.
Meanwhile, for corn salad, preheat a char-grill or barbecue over high heat. Brush cobs with olive oil and char-grill, turning occasionally, until tender (10-12 minutes). Remove from heat and, when cool enough to handle, cut kernels from cobs and place in a bowl. Separate and coarsely tear cos leaves, add to corn, then cut heart into wedges and add to corn mixture with mint. Refrigerate until required.
Meanwhile, for buttermilk dressing, whisk ingredients in a bowl until smooth, season to taste, refrigerate until required. Just before serving, drizzle a little dressing over salad, tossing lightly to combine.
Drain chicken (reserve marinade) and char-grill, turning frequently and basting with marinade, until golden brown and cooked through (25-30 minutes). Brush spring onion and asparagus with olive oil and char-grill until wilted and tender (2-3 minutes). Serve chicken and spring onion hot with corn and spring onion salad and extra buttermilk dressing.