Season chicken breasts with olive oil, salt, and pepper.
Grill or pan-sear chicken over medium-high heat for 6 to 7 minutes per side until the internal temperature reaches 165 degrees Fahrenheit (74 degrees Celsius).
Remove chicken from heat and allow to rest for 5 minutes before slicing into thin strips.
In a large mixing bowl, combine the baby spinach, sliced strawberries, toasted pecans, and thinly sliced red onion.
Add the sliced chicken strips to the salad mixture.
Drizzle the balsamic vinaigrette over the ingredients and toss gently to ensure even coating.
Sprinkle crumbled feta cheese over the top and serve immediately.