For the custard: Break the eggs into a bowl and whisk together. Add the sugar, cream, milk, salt, cinnamon and nutmeg. Whisk until the sugar is dissolved.
Trim the crust from the brioche, then cut it into a 4-by-6-inch cylinder. Place the brioche in a baking dish and cover with the custard. Let stand until the custard has soaked through all of the bread, at least 15 minutes; turn the bread occasionally with tongs to make sure it soaks evenly.
For the bananas: While the bread is soaking, peel and slice the bananas 1/2-inch thick. In a bowl, toss the bananas with the melted butter, pecans, brown sugar, lemon juice, cinnamon and 1/2 cup water and pour into a roasting pan.
For the French toast roast: When the bread has finished soaking, melt the 2 tablespoons butter in a 10-inch nonstick pan over medium-high heat. Sprinkle the soaked brioche with sugar and carefully place it, sugar side down, in the melted butter. Sprinkle the top with more sugar and sear the bread on all sides until golden brown, 1 to 2 minutes per side. Carefully place the bread over the bananas and transfer to the oven for 15 minutes, or until the bread is caramelized. Remove from the oven, dust with powdered sugar, cut into 1-inch slices and serve with the banana mixture.