Heat a large pan over medium-high heat. Add oil and onion and cook until onion wilts.
Add greens (chop the kale or collards but not the spinach) and olives, if using them.
Sauté the greens 8–10 minutes. (If using spinach, add after 5 minutes and sauté spinach for only 2–3 minutes).
Add lemon juice, salt and pepper and cook one more minute.