Remove the tomatoes from the vine, then wash and coarsely chop. Mix the chopped tomatoes with the finely sliced onions, garlic and basil, being careful not to break up the tomatoes.
Add the balsamic vinegar and olive oil. Season to taste and mix the contents again. Cover and stand aside for 30 minutes at room temperature to allow the flavours to soak and blend together.
Thickly slice the baguette diagonally and lightly toast. Generously apply the mixture to the top of the warm toast and serve.