Mix together the crab meat, tomatoes and red onion. Keep in the fridge for up to 2 days. On the day, mix through the lime zest, juice and chopped coriander, then pile onto the crostini to serve.
For the crostini bases, cut each of the 4 baguettes into 20 slices, ½cm thick, discarding ends. Drizzle with olive oil and place under the grill. Cook for 2-3 mins until lightly golden, then turn over and cook for another 1-2 mins until golden and crisp all over. Do this in batches and store in a large, airtight container at room temperature for up to 3 days.