In a very large skillet (using a soup pot is fine, too), heat the oil over medium-low heat.

Add the garlic and cook, stirring, until it softens and turns light golden, 2 to 3 minutes. 

Increase the heat to medium and add the spinach in batches, cooking down and wilting between batches, until it’s all wilted but still bright green. 

Add the red pepper flakes with the last batch of spinach. Season with salt and black pepper to taste. 

Transfer to a bowl and serve.