Grate the raw beetroot. (The red of the beetroot ensures the vegetables tossed with the vinaigrette stay red.)
Slice the onion.
Slice or chop the tomatoes.
Toss the vegetables with the vinaigrette.
Wash the lettuce. (If you can find red leaf lettuce it would do just fine as well). Place the tossed vegetables on a bed of lettuce. Cover and refrigerate at least one hour before serving.