Remove stems from from spinach leaves and discard, if desired.
Add oil and onion to a large skillet over medium heat. Sauté for 3-4 minutes, or until onions are translucent.
Stir in the rest of the spinach leaves, salt, and pepper and continue cooking for 2-3 minutes.
Serve immediately with a squeeze of lemon juice and enjoy!
To Store: Keep leftovers in an airtight container in the refrigerator for up to 3 days.
To Freeze: Store in a freezer-safe, airtight containers and freeze for up to 3 months. Add to soups, stews, or dips!