Science of Cooking

Saved you a click: Yes, you can reheat spinach. The myth that its nitrites may reform into nitrosamines only happens at high heat, so maybe cook a bit slower, but regardless, you'll be fine.

If you have a diet rich in antioxidants, which come from foods like leafy greens, they will help prevent nitrates from converting into nitrosamines.

Eating your spinach in the first place can protect you from the unlikely possible harm of reheating it again later.

Finally, it is only very high heat that will convert naturally occurring nitrates into nitrosamines, so avoid burning your spinach to a crisp, and you will be absolutely fine.