To make the salsa, combine all ingredients in a bowl. Season with salt and pepper. Stir to combine. Set aside.
Pat prawns dry with paper towel. In a large bowl, mix oil, lemon juice, garlic powder, chilli flakes and paprika. Season with salt and pepper. Add prawns and toss to combine.
Preheat a greased char-grill pan over medium high heat. Char-grill the lemon. Transfer to a plate. Drain marinade from the prawns. Cook the prawns, in batches, for about 2 minutes on each side or until lightly charred and just cooked through. Scatter prawns with green onion.
Serve with the charred lemon wedges and the salsa.