In a large baking dish, whisk together eggs, milk, three tablespoons agave, vanilla, cinnamon, nutmeg and salt. Add bread, one slice at a time, turning to coat, until all the slices fit in the pan in one or two layers. Press down with the back of a spatula to help the bread soak up the liquid. Cover with plastic wrap and refrigerate overnight.

Preheat oven to 400 Fahrenheit. Spray a large baking sheet with cooking spray. Arrange bread in a single layer on the sheet. Cook 14 to 16 minutes until golden, turning once halfway through.

While the toast is baking, combine blueberries, juice and remaining teaspoon of agave nectar in a small saucepan. Cover and cook over medium low heat. Stir the sauce after about five minutes. Cook the sauce for three more minutes, until the berries burst and make some juice. Turn off the stove and let the sauce stand uncovered for at least three minutes so the sauce can get thick. Serve blueberry sauce with the hot French toast.