Tuna salad with soused onions

Ingredients

• 2 tablespoons capers, packed in salt
• 1 medium red onion, thinly sliced
• 4 tablespoons extra virgin olive oil
• 1 1/2 tablespoon red wine vinegar
• 1 clove garlic, crushed
• 1/4 cup small black olives (optional)
• 150 grams tuna loin
• 1 tablespoon parsley, chopped
• 1/4 teaspoon salt
• 100 grams wild rocket, trimmed
• bread, to serve (omit if eating gluten-free)
• olives, to serve