Line bottom of prepared pan with 6 slices bread. Sprinkle with cheese and herbs. Top with remaining bread slices.
In a medium bowl, whisk together eggs, milk, half-and-half, mustard, salt, and pepper until frothy; slowly pour onto bread. Cover and refrigerate overnight.
Preheat oven to 350°. Uncover casserole, and let stand at room temperature for 30 minutes.
Bake until puffed and center is set, about 1 hour. Let stand for 10 minutes before serving. Garnish with chives and basil, if desired.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/breakfast-casserole-recipe/