Combine all of the ingredients in a small bowl until well combined. Taste and adjust the salt. Add a little extra rice vinegar if you find the tomatoes particularly sweet.Mash the avocado flesh to a fairly smooth pulp.Stir in the remaining ingredients.Give it a taste and add salt and lemon juice to taste. You are looking to taste enough of the lemon juice to be able to identify it along with the avocado, rather than it simply providing acidity.Lightly toast the sourdough bread.Top each piece with an equal amount of the avocado mix, spreading it out to the edges.Pile the tomato salad on top of the avocado