It was probably just fresh, house-made mayo instead of the white goo from a jar. I've never heard of "French Mayo" as a style, and I've spent some time in France, eaten at many a nice place in Paris and the countryside. Maybe it's one they've used Dijon in for the mustard part. Makes sense.

I make my own, and let me tell you it's not the same thing as jar stuff. Mine is tangy, while jar stuff is flat (thanks pasteurization). Easy to make with a $20 stick blender.

Also, are you sure it was mayo and not aioli? They look similar, but aioli is made from garlic and olive oil, and is a common side for fries. It's delicious on fries.

There's a few different kinds of popular mayonnaise in France you may be referring to: the hard Dijon flavored stuff, or the type with more vinegar (used a lot for fries).

This brand makes some great stuff: https://maille.com

One egg yoke, 3 tea spoons of fine French Dijon mustard, salt and sunflower oil is all you need for French mayonaise. Have everything at room temperature, it is important. Also, I recommend using a whisk, it will have a better firmness than with a stick blender. Whisk the egg yoke, salt and mustard. Add the oil very carefully, and don't whisk too fast.

Common French brand mayonnaise:

Amora mayonnaise Bénédicta mayonnaise

Absolutely seconding making one’s own mayo. I do keep Duke’s mayo in the fridge for ease of use though. Busy with kindergartner/shift work/etc. Duke’s has a decent acidic zing that a lot of the other brands don’t have (imo)

Edit to add, some folks will even use pickle juice in lieu of their vinegar. Don’t be afraid to use different types of vinegar as well. I like to use apple cider vinegar or legit Spanish sherry vinegar when I’m feeling fancy.

You can make mayonnaise with relative ease at home using a blender or food processor. If you can figure out the name of the specific type of mayonnaise, it would likely be easier to make it from scratch than finding it ready-made at the store.

I'm making a few assumptions here ofc, but the main downside that comes to mind is how long it'd keep for, as you wouldn't have much if any preservatives in the homemade product, though can't say I'm experienced with that at all to properly comment.

I'm French. I always do my own mayonnaise. You take one egg yolk and one small table sppon of mustard (French one, the US one tastes really differently). Mix this with a whisk. Then gradually add 250 ml of oil (sun flower for example) and whisk it at the same time. Do it really slowely. That's all you have to do.

The difference is vinegar. Idk why but most American mayos are sweet instead of tart. Acidic mayo is way better imo. They keep trying te sell Hellman's over here but barely anyone buys them.