Egg Salad Sandwich with Avocado and Watercress

INGREDIENTS

• 8 slices whole grain bread, toasted
• 1/4 cup mayonnaise, plus extra, if you like, for spreading
• Salt and freshly ground black pepper
• 1/2 bunch watercress, stems trimmed
• 6 eggs
• 1 teaspoon Dijon mustard
• 1 avocado, halved, peeled, seeded and sliced lengthwise
• 1/2 lemon, juiced