In a large bowl, whisk together lemon juice, 4 Tbsp. oil, 1/2 teas. salt, 1/2 teas. pepper, and onion powder. Add chicken, turning to coat.
Cover and refrigerate for at least 2 hours and up to 6 hours.
Remove chicken from bowl and discard marinade.
Pat chicken dry. Brush with remaining 2 Tbsp. oil. Sprinkle with remaining salt and pepper.
Spray grill rack with cooking spray. Preheat grill to 350-400 degrees.
Grill chicken to internal temperature of 165 degrees, appx. 4-5 minutes per side.
Let cool for 20 minutes.
On a large platter, arrange lettuces, strawberries, tomato, and cucumber.
Slice chicken (may be warm or cold) and place over lettuce mixture. Sprinkle basil and red onion over salad.
Serve with Strawberry Vinaigrette.