Season the chicken breast evenly on both sides with sea salt, black pepper, dried oregano, and garlic powder.
Heat half of the olive oil in a non-stick skillet over medium-high heat.
Place the chicken in the skillet and sear for 5 to 7 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the remaining olive oil, lemon juice, and Dijon mustard in a small bowl to create the dressing.
In a large salad bowl, combine the mixed baby greens, sliced cucumber, and halved cherry tomatoes.
Remove the chicken from the pan and let it rest for 3 minutes before slicing into thin strips.
Top the salad with the sliced chicken and avocado, drizzle with the lemon-herb dressing, and serve immediately.