Boil and peel the potatoes. Boil and peel the egg. Rinse tomatoes and lettuce. Open cans of tuna. Drain the liquid from the jar.
Start laying out the salad. Place lettuce leaves on a large, wide plate. Make the leaves smaller by dividing them into several parts. Place boiled potatoes on the salad.
Cut the tomatoes and egg into small round slices. Place egg and tomato rounds on the salad. Mash the tuna pieces with a fork. Distribute it evenly over the salad. Top the salad with grainy mustard and lemon juice. Add salt to taste. Add some pepper if you like.
Garnish the finished salad with avocado slices. This savory, buttery fruit goes well with fish and also makes a salad even healthier.