Creamy Chicken Mushroom Salad 'Sherlock' (Keto, Whole30)

Ingredients

• 3 hard-boiled eggs (diced)
• 1.5-2 tablespoons olive oil
• 1 medium onion (finely diced)
• 110 g walnuts (chopped (4 oz.))
• 1/3 cup mayonnaise
• 250 g cooked chicken (shredded or diced into cubes (8-9 oz.))
• 160-200 g marinated mushrooms (sliced - see notes (6-7 oz.))
• 1/2 teaspoon salt
• 1/4 teaspoon pepper
• 1 teaspoon Dijon or yellow mustard
• 1 garlic clove (crushed or minced (optional))
• 1 spring onion (chopped for final garnish (optional))
• Cos or gem lettuce leaves to serve with