1. In a large bowl, whisk together 1/4 cup oil and lemon juice; season with salt and pepper. Add zucchini; toss to coat, and let marinate while cooking chicken.
2. In a large nonstick skillet, heat remaining tablespoon oil over medium. Season chicken with salt and pepper. Cook chicken until golden brown and opaque throughout, about 7 minutes per side. Remove from skillet, and slice thinly.
3. Toss chicken with zucchini mixture, spinach, onion and cucumber.