In a Dutch oven or similar large, deep pot, add the oil and butter over medium heat.
As soon as the butter begins to bubble and foam, stir in the garlic.
Add the spinach by big handfuls, stirring as you go so that it begins to wilt down and you can fit more in the pot.
Once all of the spinach has been added, cover the pot and reduce the heat to medium low. Let cook 3 minutes. Uncover and stir. The spinach should be fairly wilted, but if it’s not, continue to stir until it is.
Stir in the salt, 1 tablespoon of the Parmesan, and the red pepper flakes. Taste and adjust the seasoning as desired. With a slotted spoon, remove it to a serving bowl. Sprinkle with the remaining 1 tablespoon Parmesan. Enjoy immediately.
TO STORE: Refrigerate spinach in an airtight storage container for up to 4 days.
TO REHEAT: Gently reheat spinach in the microwave until warm.
TO FREEZE: Pack the spinach leaves into the wells of an ice cube tray. Freeze until solid. Transfer the frozen spinach cubes to an airtight freezer-safe storage container for up to 3 months. Thaw the cubes in the refrigerator. While frozen sautéed spinach isn't ideal for thawing and enjoying on its own, it works beautifully as an addition to other recipes. Try incorporating it into your favorite soups, pastas, pizzas, sandwiches, and more.