Add the olive oil to a large, deep skillet and place over medium heat. Add the garlic and cook, stirring, for a minute or two or just until fragrant. Reduce heat if needed to avoid burning the garlic.

Add the spinach, a handful at a time, packing it down into the pan. Use tongs to move the spinach around and toss it in the garlic olive oil. Cover the skillet briefly, about 30 seconds, just until the spinach begins to cook down. Remove the cover and toss again.

Remove from heat just as soon as the spinach is mostly wilted. Pour off excess liquid from the skillet if needed. Season with salt and pepper to taste and add the lemon juice. Toss well to combine. Sprinkle with crushed red pepper flakes if desired, and serve immediately.