Tuna Provencal

• 2 tomatoes (plum)
• 1/2 onion (red minced)
• 30 ml wine vinegar (red)
• 16 ml olive oil
• 9 gr capers
• 12 slices cured black olives (sliced)
• 6 slices bread (whole grain)
• 1 package tuna (white drained)
• some salt and pepper
• some watercress