Season the chicken breast evenly on both sides with sea salt, black pepper, and dried oregano.
Heat a non-stick skillet over medium-high heat and cook the chicken for 6-7 minutes per side, or until the internal temperature reaches 165°F.
While the chicken cooks, prepare the dressing by whisking together the olive oil, lemon juice, and Dijon mustard in a small bowl until emulsified.
Dice the cucumber, halve the cherry tomatoes, and thinly slice the red onion.
In a large bowl, toss the mixed greens, cucumber, tomatoes, and red onion with the prepared lemon vinaigrette.
Allow the chicken to rest for 3 minutes after cooking, then slice it into thin strips.
Top the salad with the sliced chicken, fresh avocado slices, and a garnish of chopped fresh parsley.