Preheat oven to 425 degrees. Place sliced sourdough on a foil covered baking sheet and brush the bread with a small amount of olive oil. Bake 8-10 minutes until lightly toasted. Remove from oven and cool.
In a large sauté pan, add 2 T olive oil and 2 T of the butter and cook on low heat until butter is melted. Add sliced yellow onions and continue to cook on low heat until caramelized, about 25 minutes. Place onions in separate bowl to cool. Add the remaining 1 T of butter to the pan and sauté pears for about 5 minutes or until tender. Combine the balsamic vinegar, honey or maple syrup with the onions and add salt and pepper to taste
Place a small amount of blue cheese on top of each crostini, followed by a spoonful of the onion mixture, then pear slices and finally the chopped walnuts. Return the crostini to the oven and bake for about 4 minutes. Remove and serve warm. Serves 8-10.