Season the chicken breast evenly with half of the sea salt, black pepper, and dried oregano.
Heat a grill pan or outdoor grill over medium-high heat and cook the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
While the chicken is grilling, whisk together the extra virgin olive oil, lemon juice, raw honey, and Dijon mustard in a small bowl with the remaining salt, pepper, and oregano.
In a large salad bowl, combine the mixed baby greens, sliced strawberries, and blueberries.
Remove the chicken from the grill and let it rest for 3 minutes before slicing it into thin strips.
Drizzle the lemon-honey vinaigrette over the salad greens and berries, tossing gently to coat.
Top the salad with the sliced grilled chicken, chopped walnuts, and crumbled feta cheese before serving.