Begin by preparing the ingredients. Check the smoked mackerel fillet for any bones and break the fish with your hands into chunky, bite-sized pieces. Halve the avocado, remove the pit, scoop the flesh from the skin, and cut it into even cubes. Let the roasted red pepper drain well, then slice it with a sharp knife into thin strips.
Next, prepare the vinaigrette. Whisk the olive oil and apple cider vinegar together vigorously in a small bowl until smooth. Season the mixture to taste with some ground pepper. Use a large platter or divide the salad directly among the plates. Layer the mixed greens as a light base on the bottom.
Distribute the flaked mackerel pieces, avocado cubes, and pepper strips evenly over the bed of greens. Drizzle the prepared vinaigrette over the ingredients so everything is subtly coated with dressing. Finally, chop or pluck the dill finely and scatter it as garnish over the top. Serve the salad immediately.