Preheat the oven to 250⁰F. Place a cooking wire rack inside an 18-by-13-inch sheet pan; set aside. Slice the bread into ¾-inch thick slices and set aside.
In a medium bowl, combine evaporated milk, eggs, vanilla extract, nutmeg, cinnamon, and brown sugar. Whisk until the mixture is somewhat homogenous, about 1 minute. Pour into a shallow dish.
Place a few slices of the bread in the custard mixture and let it soak, occasionally flipping until saturated but not falling apart, about 1 to 2 minutes, depending on how moist you like your French toast (try not to over-saturate).
Heat 1-½ tablespoons avocado oil and 1-½ tablespoons of unsalted butter in a very large saute pan over medium heat; swirl the melted butter around the pan; when it starts to get golden brown, it's ready for the bread. Add the soaked bread and cook for 3 to 4 minutes on each side until nicely browned.
Place the cooked French toast on a sheet pan and keep it warm in the oven. Fry the remaining soaked bread slices, adding unsalted butter and avocado oil as needed until it's all cooked. Serve hot with maple syrup, fruit preserves, and/or confectioners' sugar.