Preheat oven to 250°F.
Add eggs, half and half and vanilla to a small bowl. Whisk until well-blended.
Add brown sugar, cinnamon and salt. Whisk until combined.
Pour mixture into an 8-inch square baking dish. Whisk in 2 tablespoons brown sugar, cinnamon, nutmeg (if using) and salt.
Melt 2 tablespoons butter in a large skillet over medium heat.
Dip a slice of banana bread in the egg mixture. Turn to coat both sides.
Place bread in the hot pan. Cook on each side until golden brown.
Remove from pan to a cooking sheet. Place in preheated oven to keep warm.
Repeat with remaining slices of bread.
When all slices are cooked, wipe the skillet clean with a paper towel.
Add the remaining 2 tablespoons butter.
When butter has melted and is hot, add remaining 2 tablespoons brown sugar and lemon juice. Cook and stir until smooth.
Add banana slices. Toss until glazed, about 1 minute. Remove from heat.
Cut french toast slices in 1/2 diagonally. Place 2 or 4 pieces on a plate.
Drizzle with prepared banana caramel sauce, making sure to include a few banana slices.
Dust with powdered sugar and sprinkle with chopped walnuts.
Top with whipped cream, if desired.
Serve as is or with a syrup of choice.