Heat barbecue or chargrill plate over high heat. Toast sourdough for a couple of mins on either side, until crisp. Remove from heat.
Place lemon juice and caster sugar in large mixing bowl and whisk to dissolve sugar. Add extra virgin olive oil and a little salt and pepper; whisk to combine.
Tear toast slices and add to dressing in bowl. Add tomatoes and spring onions and toss well with your hands to combine, squeezing a little to release some of the tomato juices. Season with salt and pepper.
Place prawns in medium mixing bowl, add lemon zest and season. Toss to coat. Cook on barbecue for a few mins, until just cooked through.
Remove prawns from barbecue and add to bowl containing bread and tomatoes. Using spoon, scoop flesh from avocados and add to bowl. Add basil and a splash of balsamic vinegar and toss well to coat. Serve while warm.