Season chicken breasts with salt and pepper; grill over medium-high heat for 6-7 minutes per side or until the internal temperature reaches 165°F (74°C).
Allow the chicken to rest for 5 minutes, then slice against the grain into thin strips.
In a blender or tight-sealing jar, combine vegetable oil, white vinegar, sugar, water, orange juice, onion powder, dry mustard, and salt; pulse or shake vigorously until emulsified.
Stir the poppy seeds into the dressing manually and set aside.
In a large serving bowl, combine the chopped romaine lettuce with the sliced strawberries, blueberries, pineapple chunks, and drained mandarin oranges.
Top the fruit and lettuce mixture with the grilled chicken strips and toasted pecans.
Drizzle the poppyseed dressing over the salad and toss gently to coat immediately before serving.