First, preheat oven to 350F. Spray a 9×13 pan with nonstick spray. Cut each slice of bread into 9 squares, 3×3 and evenly layer it into the baking dish.
In a medium-sized bowl, whisk together eggs, eggnog, yogurt, agave, cinnamon, nutmeg, and vanilla. Pour the eggnog mixture over top of the bread, making sure all bread pieces are covered. Next, cover the pan with plastic wrap and refrigerate for 6-8 hours or overnight.
After sitting for 6-8 hours or overnight, place the french toast bake in the oven for 30 minutes. While this bakes, mix together walnuts, ½ cup brown sugar, and 1 tablespoon butter. Microwave for 1 minute and stir.
After 30 minutes remove the french toast bake from the oven. Sprinkle the walnut and brown sugar mixture over top. Place pan back in the oven for an additional 15 minutes.
Enjoy with fresh berries!