Preheat oven to 350°F. Place almonds on a foil or parchment-lined cookie sheet in a single layer. When oven is heated, place cookie sheet with almonds inside, and turn off the heat. Let almonds stand in the oven for five minutes or until lightly toasted. Remove from oven, set aside.
Cut chicken into chunks, set aside.
Quarter the onion and grate or process to a fine puree. Mix with mayonnaise, mustard, lemon juice or white wine vinegar. Fold in sliced celery and herbs. Add salt and freshly ground pepper to taste.
Fold sauce into reserved chicken chunks and taste for seasoning. If mixture seems dry, add a bit more mayonnaise. Chill until ready to serve.
Serve topped with almonds, reserving some almonds to pass separately, for those who like to add more.