Preheat oven to 450 degrees. Pound out chicken breasts so they are even in thickness. Place on a foil lined baking sheet.
Melt butter and brush over the chicken. Season with salt, pepper, garlic powder and paprika, on both sides. Bake for 15 minutes or until no longer pink in the center. Allow chicken to sit for 5 minutes before slicing.
Put arugula and spinach in a mixing bowl. Slice strawberries and add to greens. Thinly slice onions and add to the bowl along with feta crumbles.
Sprinkle balsamic vinegar over the salad, drizzle with olive oil and season with salt. Toss to coat evenly.