Preheat oven to 450°F. Mist a large baking dish with olive oil cooking spray.
In a steamer basket over simmering water to medium, steam baby spinach until wilted, 1 to 2 minutes. Let cool, squeeze out water and chop.
In a large bowl, combine lean ground chicken, minced garlic cloves, lightly beaten large egg, whole-grain bread crumbs, crumbled feta cheese, finely chopped fresh basil leaves, sea salt and fresh ground black pepper, to taste, and spinach; mix well to combine. Use your hands to form mixture into 12 balls.
Transfer to prepared baking dish and bake for 15 to 20 minutes, until golden brown and no longer pink inside.