In a mortar and pestle, crush the garlic. Add the pepper and salt and continue mashing until mixture becomes a thick paste. Transfer to a jar along with the lemon juice and canola oil. Shake well.
In a large bowl, toss together the lettuce, tomatoes and onion. Just before serving, add the cheese and just enough dressing to coat. (There will be extra dressing, but it keeps well.) Toss well.
