Have you ever had a croissant bun? Other than the ones we make for La Boulange, you can't find them anywhere, so it's not really a fair question, and this photo is a bit of a tease. A brioche bun, any kind of soft burger bun, or the traditional hot dog roll works well with this sandwich.
Finely chop the celery and tarragon leaves. Mix together the celery, lemon zest and juice, tarragon, aioli, and celery rémoulade. Spread the celery mix on the bottom half of the bun, and then add the lobster. Cover it with the top of the bun.
Cut the ends off each celery root and peel. Shred both celery roots with a hand grater and transfer to a serving bowl. Combine the lemon juice and mayonnaise. Pour over the celery root and toss.