This was one of those last minute, "we need a salad, let's see what we can make with what we have" recipes. Luckily it turned out great! It's flavorful, simple, elegant, and surprisingly beautiful to look at. —Natalie
about 1 1/3 c mayonnaise, filing more than 10 avocado halves
Whisk together egg yolk, lemon juice, and vinegar until combined.
Slowly whisk in the oil, drizzled in a very thin stream, beginning a few drops at a time. Continue very slowly drizzling in the oil, whisking all the while, until all the oil is combined and the mayonnaise is very thick (this could take 10 minutes).
If at any point you add too much oil, stop pouring and whisk the mayonnaise mixture until the oil is fully incorporated. Then resume the slow addition of oil while whisking.
Once the mayonnaise is completely emulsified, whisk in the shallot, tarragon, salt and black pepper. Taste for seasoning, and add more salt and/or pepper as necessary.
Refrigerate in a covered container at least an hour so the flavors meld.
Fill halved and pitted avocados with the shallot and tarragon mayonnaise and serve.
Mayonnaise can be safely stored in the refrigerator about 1 week.