Wash the spinach thoroughly. Cook in a saucepan over low heat about 5 minutes; do not add water. (There is usually enough water already clinging to the leaves to steam the greens.) Collard greens or kale may take 7 - 10 minutes.
Heat the oil in a skillet and sauté the onion and tomato for 4 - 5 minutes. Add the cooked greens, lemon slices, and seasonings; stir constantly for a few minutes. Serve hot.