Tuna Salad Sandwich

• 95 g tin tuna, in olive oil
• 1 tbsp Greek yoghurt
• .25 red onion, diced
• 1 tsp seeded mustard
• .5 Lebanese cucumber, sliced
• 2 cos lettuce leaves
• 1 beetroot, cooked and sliced
• .5 tomato, sliced
• 2 slices grain bread