Add onion and sauté 2-3 minutes until onion starts to turn translucent.
Add spinach, salt, pepper and garlic and stir to coat spinach in oil. Continue stirring every 30 seconds until spinach is wilted (about 4-5 mins). Do not continue cooking once spinach is done wilting - immediately remove from heat.
Add lemon juice and red pepper flakes (if desired) and stir. Transfer to a serving dish with a slotted spoon to drain out the juices.