In a large skillet, heat the olive oil over medium. Once hot, add the onion and cook, stirring often, until softened, about 4 minutes. Add the garlic and cook until fragrant, about 30 additional seconds.
Add the chicken, increase the heat to medium high, and let cook for 3 minutes, browning all sides. Stir in the soy sauce and black pepper. Let cook until the chicken is completely cooked through, about 3 minutes longer.
Stir in the spinach a few handfuls at a time, letting the heat of the pan wilt it as you go. Stir in the lemon zest, lemon juice, and basil. Cook and stir just until the basil is wilted, about 1 additional minute. Taste and season with additional salt or pepper as desired. Serve warm with rice as desired.