The Ice Cream Scoop

I have a friend that I will occasionally share a dessert with when we're out at a restaurant. In recent months, we've discovered a strange phenomenon: waiters who don't know what à la mode means. For those who don't know (why not?!?) to order something à la mode means to order it with a scoop of vanilla ice cream on top. The first time there was confusion over the phrase à la mode I found it funny. How amusing that the waiter had never heard that before. After all, isn't it a part of the American English lexicon? There's a whole moment devoted to it in When Harry Met Sally:

Sally: I'd like the chef salad, please, with the oil and vinegar on the side. And the apple pie à la mode....But I'd like the pie heated, and I don't want the ice cream on top. I want it on the side. And I'd like strawberry instead of vanilla if you have it. If not, then no ice cream, just whipped cream, but only if it's real. If it's out of a can, then nothing.
Waitress: Not even the pie?
Sally: No, just the pie. But then not heated.

When a second waiter was confused over à la mode, I found it downright disconcerting. Has à la mode fallen out of use? How did it come to its meaning in the first place?

First let me warn you that I found no reliable sources for what I'm about to share with you, so I hope you'll forgive me for an unusual lack of citations.

As any French speaker will tell you, the literal translation of à la mode is "of the fashion." It's generally used to indicate that something is particularly fashionable. It has been suggested that the American English usage originates from the phrase à la mode de (insert town or region) to instead mean "as they do it in (town or region)." In other words, you would be asking the waiter to have it prepared as it is prepared in a particular place.

However, that still doesn't explain how the phrase came to be used in the United States, and came to mean "with ice cream." The only explanation I could find was a mention that the famous New York restaurant Delmonico's perhaps first added "apple pie à la mode" to their menu in the 1920s. It being an incredibly fashionable restaurant, perhaps the usage caught on from there.

I'm sure if I looked beyond the internet, I could track down a more accurate source of the American use of the phrase à la mode. In the meantime, I should probably just take it as a sign that my friend and I should be ordering less dessert à la mode for us to share.