Using a large pan (13 x 9,) lightly oil the bottom of the pan and the sides with olive oil. Place chicken breasts side by side in the pan (using one half breast per person.) Evenly distribute creme fraiche over the chicken breasts and bake in a 375 - 400 degree oven for around 45 minutes. Make sure that the chicken breasts are fully cooked when you remove them from the oven. Allow the chicken breasts to cool completely. Meanwhile finely chop the celery and slice the grapes in half, and put them together in a bowl and place them aside. With the remaining creme fraiche and sour cream, mix them together thoroughly in a bowl. If you want to add a bit of zip to the flavor, you can add the dijon mustard. If you don't want as much flavor from the mustard, you can add 1/4 cup instead of half. Mix all of this together thoroughly and put that aside. When the chicken is fully cooled, chop the chicken up into small bite size pieces and put it in a large bowl. Add the celery and grapes and then the sour cream and creme fraiche mixture. Mix together thoroughly. Add salt and pepper to taste. To serve for company, you can sprinkle the top with some of the slivered almonds and grape halves and then add sprigs of parsley.