Season the chicken breast evenly with half of the sea salt, half of the black pepper, and all of the dried oregano.
Heat 0.5 tbsp of olive oil in a medium skillet over medium-high heat until shimmering.
Add the chicken to the skillet and sear for 6 to 7 minutes per side until the exterior is golden brown and the internal temperature reaches 165 degrees F.
While the chicken is cooking, whisk together the remaining 0.5 tbsp olive oil, lemon juice, minced garlic, and the remaining salt and pepper in a small bowl to create the dressing.
In a large salad bowl, combine the mixed greens, halved cherry tomatoes, and sliced cucumber.
Remove the chicken from the pan, let it rest for 3 minutes, then slice it into thin strips.
Arrange the sliced chicken and avocado over the salad greens and drizzle the lemon herb vinaigrette over the top before serving.