Pat the chicken breast dry with paper towels and season both sides evenly with the sea salt and black pepper.
Preheat a grill or grill pan over medium-high heat and lightly coat with a touch of oil to prevent sticking.
Grill the chicken for 6-7 minutes per side, or until the internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, place the walnuts in a small dry skillet over medium heat and toast for 3 minutes until fragrant and lightly browned.
In a small bowl, whisk together the extra virgin olive oil, balsamic vinegar, and Dijon mustard until the dressing is emulsified.
Wash and slice the strawberries, and rinse the blueberries and mixed greens.
In a large bowl, toss the mixed greens with half of the balsamic dressing.
Remove the chicken from the grill and let it rest for 5 minutes before slicing into thin strips.
Top the greens with the sliced chicken, strawberries, blueberries, and toasted walnuts.
Drizzle the remaining dressing over the top and serve immediately.