I didn't think much more of it until fairly recently when I got on an anchovy kick. I mentally combined the ingredients I could remember: garlic, olive oil, anchovies,...cream?...and it was good. Even though the cream seemed a bit excessive. I wanted to learn more.
I did the minimum research required for any such question - I looked it up on Wikipedia.
The strength of Wikipedia isn't in the information it gives you, which is usually freely editable and can be modified by anyone, regardless of how little they know about the subject. (Wikipedia actually discourages anyone who is a subject matter expert from contributing to a topic.) The real strength lies in the references. Wikipedia can be very useful as a jumping-off point to other sources of information, sources that are professionally edited and reflect the trustworthiness of the groups with which they are associated. A footnote on the Wikipedia entry led me to this article from the New York Times Magazine:
This was originally nothing fancy, a simple sauce or fondue used to feed vineyard workers in the Winter. Olive oil, garlic, and anchovies, slowly heated together, used as a dip or sauce for vegetables - or, as Michael Garibaldi had it on Babylon 5, even with bread. (It's great with bread.) The cream and butter are regional additions and probably change the taste and character substantially Walnut oil, salt-dried anchovies, fermented black garlic - make it as plain or fancy as you like. It's your kitchen, and you (and your friends and family) will be the one eating it.
For me, the recipe is simple, and involves just three ingredients.
In a small saucepan I place however much extra-virgin olive oil as I feel like - maybe half a cup, if this is going to make multiple servings. I turn the heat to medium and add some garlic: a few large cloves chopped up, or many long, thin ones with the ends removed. (These can be broken up further.) I then add one tin of King Oscar flat anchovies in olive oil, pouring the residual oil in for added anchovy goodness (and, of course, the dissolved salt.) Soon the oil will begin to bubble around the garlic cloves and the anchovies, and this is the sign that you should reduce the heat to its lowest setting and go surf the Internet for a while.
After about ten or fifteen minutes you can check to see that - ta-daaa! Your anchovies are gone! They haven't dissolved, but they have disintegrated into a powder that is mostly congregated at the bottom of the saucepan. Your olive oil will also no longer be yellow-green but will have taken on a brownish tinge from the anchovies. At this point try to squish the garlic with a fork. If it's soft and squishy, you're good. But if it's still hard, it needs some more time to simmer.
In either case, you can get your vegetables and/or bread ready now. I like my bagna cauda with steamed broccoli, but you can have whatever you like. Traditionally the vegetables should be dipped in the sauce as it simmers, but I drizzle it over the vegetables and enjoy it like that. Then I take slices of Maier's Italian Bread, sometimes cut into triangles, and sop up the residual sauce.
As this is made with olive oil and fish, it qualifies as a health food and will cause you to live forever.*
*Preceding statement has not been tested or verified by the U.S. Food and Drug Administration.