Season the chicken breast evenly with half of the sea salt, black pepper, and dried oregano.
Heat half of the extra virgin olive oil in a skillet over medium-high heat.
Place the chicken in the skillet and cook for 6-7 minutes per side until the internal temperature reaches 165°F and the exterior is golden brown.
While the chicken cooks, whisk together the remaining olive oil, fresh lemon juice, Dijon mustard, and the rest of the salt, pepper, and oregano in a small bowl to create the dressing.
In a large salad bowl, combine the mixed baby greens, sliced cucumber, and cherry tomatoes.
Drizzle the lemon-herb dressing over the vegetables and toss gently to coat.
Slice the cooked chicken into thin strips and arrange them over the bed of greens.
Top the salad with the sliced avocado and serve immediately while the chicken is still warm.