Pat the chicken breast dry with a paper towel and season both sides evenly with the sea salt and black pepper.
Preheat a grill or cast-iron grill pan over medium-high heat. Once hot, grill the chicken for 5-7 minutes per side until the internal temperature reaches 165°F.
While the chicken is cooking, whisk together the extra virgin olive oil, fresh lemon juice, Dijon mustard, and honey in a small bowl until the dressing is emulsified.
In a small dry skillet over medium heat, lightly toast the walnuts for 2-3 minutes until they are fragrant and golden brown, then set aside.
Remove the chicken from the heat and let it rest for 5 minutes before slicing it into thin strips against the grain.
In a large salad bowl, toss the mixed greens with half of the prepared dressing until lightly coated.
Top the greens with the sliced strawberries, blueberries, toasted walnuts, and the grilled chicken strips.
Drizzle the remaining dressing over the top and serve immediately while the chicken is still warm.