Tuna and Egg Sandwiches (Tramezzini al Tonno e Uovo)

Ingredients

• 4 eggs, at room temperature
• 425 gm Italian canned tuna in oil, drained
• 2 golden shallots, finely chopped
• 40 gm tiny capers, rinsed
• 4 anchovy fillets, finely chopped
• 8 soft white bread slices, crusts removed
• For spreading: softened butter
• 2 cups (loosely packed) watercress
• Maionese
• 2 egg yolks
• 1 tbsp lemon juice
• 1 small garlic clove, finely chopped
• 1 tsp Dijon mustard
• 250 ml (1 cup) olive oil