Preheat oven. Preheat the oven to 350 degrees Fahrenheit.
Cook chicken. Rub olive oil on the chicken breasts and sprinkle with salt and pepper. Place chicken on a sheet pan and roast for 45-55 minutes (depending on the size of the chicken breasts) until the chicken reaches 165 degrees Fahrenheit when checked with a meat thermometer. Remove from oven and let cool.
Make the dressing. While the chicken cools, whisk together mayonnaise, Dijon mustard, vinegar, 1 tablespoon olive oil, sugar, ½ teaspoon salt, and ¼ teaspoon pepper in a small bowl. Taste for seasoning, adding more salt or sugar if desired. Cover with plastic wrap and chill in the refrigerator.
Chop chicken. Remove the skin and bones from the chicken and shred or chop the chicken into bite-size pieces.
Combine. In a large bowl, combine chicken, grapes, celery, ½ cups scallions, almonds, and cranberries. Fold in refrigerated dressing and garnish with additional scallions.
Serve. Serve chicken salad immediately in croissants with lettuce or lettuce and tomato if desired.