Season chicken breasts with salt and pepper on both sides.
Heat olive oil in a skillet over medium-high heat; cook chicken for 6 to 7 minutes per side until the internal temperature reaches 165 degrees Fahrenheit.
Remove chicken from heat and allow to rest for 5 minutes before slicing into thin strips.
In a small mixing bowl, whisk together Greek yogurt, apple cider vinegar, honey, poppyseeds, and Dijon mustard until a smooth dressing forms.
In a large salad bowl, combine the spinach and romaine mix with strawberries, blueberries, mandarin oranges, and pineapple.
Add the sliced chicken and chopped pecans to the salad base.
Drizzle the Greek yogurt poppyseed dressing over the salad components.
Toss gently to ensure even distribution and serve immediately.