Preheat the oven to 180°C (Gas 4). Brush potatoes with olive oil and pierce with fork. Sprinkle salt over them and bake for 1 hour.
Combine the tuna, mayonnaise, chives and onions. Add 50 g of Cheddar.
Cut the baked potatoes in half and scoop some of the flesh into the tuna mixture.
Mix the potato and tuna, season with salt, pepper and olive oil and fill the potato shells.
Top with the rest of the Cheddar and place in the oven until golden.
Meanwhile, chop the red onion and spring onion, mix in a bowl with the spinach and mixed lettuce. Sprinkle with olive oil, white wine vinegar and season with salt and pepper. Serve with the stuffed potatoes. Enjoy!