Season the chicken breast evenly with sea salt, black pepper, and dried oregano on both sides.
Heat a grill pan or skillet over medium-high heat and cook the chicken for 6 to 7 minutes per side until the internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, prepare the vinaigrette by whisking together the extra virgin olive oil, lemon juice, and Dijon mustard in a small bowl until emulsified.
Slice the cucumber into thin half-moons and halve the cherry tomatoes.
Thinly slice the radishes and the quarter of an avocado.
In a large serving bowl, combine the mixed greens with the cucumber, tomatoes, and radishes.
Allow the grilled chicken to rest for 3 minutes, then slice it into thin strips.
Arrange the chicken strips and avocado slices over the salad, sprinkle with pumpkin seeds, and drizzle the vinaigrette over the top before serving.