Season the chicken breast evenly on both sides with half of the sea salt, black pepper, and dried oregano.
Heat a skillet over medium-high heat with half of the olive oil and sear the chicken for 6-7 minutes per side until golden brown and the internal temperature reaches 165°F.
While the chicken is cooking, whisk together the remaining olive oil, lemon juice, Dijon mustard, and the remaining spices in a small bowl to create the vinaigrette.
Prep the vegetables by slicing the cucumber into half-moons, halving the cherry tomatoes, and thinly slicing the avocado.
Place the mixed greens in a large serving bowl and toss with the prepared cucumber and tomatoes.
Remove the chicken from the pan, let it rest for 3 minutes, then slice into thin strips.
Arrange the warm chicken strips and avocado slices over the salad, then drizzle the lemon-herb vinaigrette over the top just before serving.