Sourdough bread: 60g (1 big slice), Tuna (canned in water, drained): 85g (3 oz), Greek yogurt (plain, low-fat): 15g (1 tbsp), Mayonnaise (low-fat): 10g (2 tsp), Dijon mustard: 5g (1 tsp), Red onion: 10g (2 tsp, diced), Tomato: 30g (2 slices), Lettuce: 15g (1/2 cup, shredded), Lemon juice: 5ml (1 tsp), Salt and Black pepper: to taste