Start by toasting the ciabatta, either under a grill, in the oven or in a toaster.
In a large mixing bowl mix the onion, garlic, olive oil, lemon juice and chilli.
Now take the basil leaves, roll them up together, almost like a cigar, and slice.
Add these to the bowl, then add the chopped tomatoes.
To serve, top each ciabatta slice with some of the mix.