Season the chicken breast with half of the sea salt and black pepper.
Preheat a grill or grill pan over medium-high heat and cook the chicken for 6-7 minutes per side until fully cooked through.
While the chicken is cooking, lightly toast the walnuts in a small dry skillet over medium heat for 3 minutes until fragrant.
In a small bowl, whisk together the extra virgin olive oil, balsamic vinegar, dijon mustard, and the remaining salt and pepper to create the dressing.
Slice the strawberries and the grilled chicken into bite-sized pieces.
In a large bowl, toss the mixed greens with half of the dressing.
Top the greens with the sliced chicken, strawberries, blueberries, and toasted walnuts.
Drizzle the remaining dressing over the top and serve immediately.