How do I boil/cook frozen vegetables to maintain nutrients?
What you want to avoid is where you leave lots of liquid behind.

When you're boiling, water soluble nutrients (eg minerals, vitamins B) will be leached out.

If you cook it slowly in fat (oil, butter, etc.), but it's so much that it's a puddle left behind, you might be losing fat soluble vitamins (eg, vitamins A, D, E)

Cooking at too high of a heat can also break down nutrients into other compounds ... but it's a trade-off, as cooking breaks down cell walls and makes them more bio-available.

...

Personally, I tend to look at flavor, not just nutrients, as if it doesn't taste good, you're not going to eat it, and then you won't get any nutrients.

So, I'd recommend a 'steam-sauté' method:


Put the vegetables in a pan with a little bit of water (about 3 TB / 45mL) and a lid
Heat until you have steam, and let steam for a couple of minutes (exact time depends on the size of what you're re-heating, but you want them defrosted)
Take the lid off, and let most of the water evaporate.  You can push the veg to one side to speed this up
Add a little oil and whatever seasoning, and cook 'til you get a little bit of color


For fresh vegetables, I'll sauté first, then steam.  (and for brocolli, I saute the sliced up stems before adding the florets)