Combine 3 cups of water, white wine vinegar, sugar, fennel and coriander seeds, sea salt, lemon zest and bay leaf in saucepan and bring to the boil. Simmer for 5 mins or until sugar has dissolved. Remove from heat and let cool completely.
Place prawns in a medium non-reactive bowl and pour over pickling liquid. Transfer to the fridge to pickle overnight or for at least 4-5 hours.
Preheat oven to 100℃.
Place rock salt and thyme on a large baking tray. Top with tomatoes, season with pepper and roast for 3-4 hours.
Remove tomatoes and let cool slightly. Drain prawns from pickling liquid, reserving 2 tablespoons.
To make dressing, whisk ingredients together in a medium bowl. Add prawns and tomatoes.
Place halved baguette on a baking tray, brush with olive oil and grill for 2-4 minutes until golden.
Top with prawn and tomato mixture and sprinkle with baby basil leaves.