My 22-month daughter could stay outside for hours, and she also gets VERY upset when I just say, "Okay, time to go in now" and pick her up. You really have to give a toddler time to adjust to the idea of going in!
Now I say, "I'm getting hungry! How about you? In a few minutes we're going inside to eat. Are you going to help me? We'll put on your apron, and you can help me get the (plastic) plates and cups, and get the food ready. Doesn't that sound good?" (note: excited tone of voice is key!)
I try to make the plans sound like they're more fun than being outside...if you make it seem like a chore (ie., don't say "Sorry, we have to go in now for dinner" with a sad tone of voice) she won't want to do it. Plus, many little kids LOVE to be "big helpers," so take advantage of that!
My daughter might still be a little upset once it's actually time to go in, but when I remind her again what the plans were, she doesn't throw a tantrum.
But if she does get mad once we're inside, I remind her that we can go out again after we eat to "close up," which means "checking" that the garage, basement and car doors are locked, and taking out the garbage/recycling...it's just something we can do rather quickly, and since she knows that it's time to "close up" instead of play, she doesn't give us a hard time once the job is done and it's time to go back inside to read books and play with her dolls.
Good luck!
P.S.- This is what I do to head off tantrums. I agree that if she does end up throwing a full-blown fit, it's best to ignore it!!! You shouldn't purposely leave her alone (that just seems to make my daughter more upset...and she'll probably follow you to another room anyway) but just go about making dinner like usual.