Put the olive oil, vinegar, salt and pepper into a jar with a lid and shake together.
Slice the tomatoes into 1cm thick wedges.
Trim the top of the fennel (keep any fronds but discard the tops of the stalks). Cut in half, lengthways, on a slant. Then slice into 3mm half-moons, as though you are cutting an onion. Cut the avocados in half and slice into thin half-moons, like the fennel.
Place two layers of tomatoes on your plate, then add the fennel and avocado. Pour the dressing over your salad and finish with the basil leaves, fennel fronds and some salt and pepper.