Season the chicken breast evenly with half of the 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 until golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the extra virgin olive oil, fresh lemon juice, Dijon mustard, and the remaining salt, pepper, and oregano in a small bowl.
In a large serving bowl, combine the mixed baby greens, sliced cucumber, and halved cherry tomatoes.
Drizzle half of the lemon-herb vinaigrette over the salad greens and toss gently to coat.
Slice the cooked chicken breast into strips and arrange them on top of the salad.
Add the sliced avocado to the bowl and drizzle the remaining dressing over the chicken and avocado before serving.