Preheat oven to 375 degrees F. Soften goat cheese at room temperature.
Cut the baguette into 1/4-1/2-inch slices. Arrange on a baking sheet, brush with olive oil and season with salt to taste. Bake for 15 minutes or until golden.
Cut an onion into thin slices. To caramelized, melt 2 tbsp butter (or 1 tbsp butter, 1 tbsp olive oil) in a saucepan over medium heat. Add onions and 1 tsp balsamic and cook until browned and fragrant, stirring occasionally, about 10-15 minutes.
To candy the walnuts, melt 2 tbsp of butter in a small saucepan. Add 1/4 cup sugar and 1 cup walnuts. Coat evenly and cook for 5-8 minutes, until the mixture is browned and slightly sticky. Be careful not to burn.
Spread a dollop of softened goat cheese on each crostini. Top with caramelized onions and candied walnuts*. Serve warm.
*Candy the walnuts right before you are ready to use them. They harden as they cool which may or may not be to your liking.