Slice the quarter cucumber into rounds and halve the cherry tomatoes. Remove the skin from the half ripe avocado and cut the flesh into chunks. Check the smoked mackerel carefully for any bones and flake the fish into large pieces with your fingers.
Heat a dry skillet over medium-high heat. Toast the pumpkin seeds in it until they begin to pop and turn a light color. Remove the pan from the heat immediately and let the seeds cool briefly on a small plate.
Whisk together the extra virgin olive oil, Dijon mustard, and lemon juice in a small bowl until you have a smooth dressing. Place the mixed greens, cucumber, cherry tomatoes, and avocado in a large bowl. Pour the dressing over top and toss everything together gently but thoroughly.
Distribute the dressed salad onto a large platter or deep plate. Arrange the flaked pieces of smoked mackerel on top of the vegetables. Finally, sprinkle the toasted pumpkin seeds evenly over the dish and serve the salad immediately.