Pat the chicken breast dry with paper towels and season both sides with sea salt, black pepper, garlic powder, and dried oregano.
Heat the olive oil in a medium skillet over medium-high heat.
Place the chicken in the skillet and sear for 6 to 7 minutes per side, or until the internal temperature reaches 165°F and the exterior is golden brown.
Remove the chicken from the pan and let it rest for 5 minutes before slicing into strips.
In a small bowl, whisk together the lemon juice, extra virgin olive oil, and Dijon mustard to create the dressing.
In a large salad bowl, combine the mixed greens, sliced cucumber, and halved cherry tomatoes.
Toss the salad lightly with the prepared dressing.
Top the greens with the sliced chicken and quartered avocado, then serve immediately.