Put the mayo in a small bowl or jar. Stack the basil leaves and then roll them into a tight 'cigar'. Slice the roll very thinly with a sharp knife. Add the basil to the mayo, loosening up the ribbons with your fingers. Blend together with the lemon juice and refrigerate until needed.
Toast the bread in a toaster or in a pan on the stove. Be sure to toast both sides.
Slice the tomatoes thinly.
Spread a generous layer of mayo on each slice or bread and top with tomato slices, slightly overlapping depending on size. Sprinkle with salt and pepper and serve immediatley.