Some version of tomatoes on toast — a juicy American B.L.T. or Italian tomato-topped bruschetta — is always a good idea, but that's especially true during high summer, when tomatoes are at their peak. One superior combination comes from Barcelona, where a slice of toast is rubbed with garlic and juicy ripe tomatoes, then anointed with olive oil. Most Catalan cooks simply cut the tomato crosswise and vigorously massage the toasted bread with the cut side. Others grate the tomato flesh and…