Make the vinaigrette: in a medium bowl whisk together 1/4 cup olive oil, 1/4 cup balsamic vinegar, 1 tablespoon honey, 1 teaspoon minced garlic, 1 teaspoon red pepper flakes, and 1/2 teaspoon salt until emulsified.
Reserve about 1/4 cup of the vinaigrette for dressing, then place the chicken breasts in a bowl and pour the remaining vinaigrette over them to coat. Marinate for 30 minutes.
Heat a grill pan or grill over medium heat until hot. Grill the chicken about 6 minutes per side, or until an instant-read thermometer registers 165°F in the thickest part.
Remove the chicken and let rest until cool enough to handle, then slice into strips.
Meanwhile, in a large bowl combine 4 cups mixed salad greens, 2 cups sliced strawberries, 1 cup sweet corn, 8 ounces crumbled feta, 4 sliced radishes, 1 small thinly sliced red onion, and 2 tablespoons pumpkin seeds.
Drizzle the reserved vinaigrette over the salad and gently toss to combine; adjust seasoning to taste.
Top the salad with the sliced grilled chicken and serve.