Slice onions into thin rings. Add a dab of butter or splash of olive oil (or both) to a large pan at medium high heat. Add onions to the pan, carmelizing the onions will take approximately 40 - 45 minutes. When onions were finished I was left with just over one cup of carmelized onions.
Dice pears to small cubes, approximate 1/4 inch in size.
Chop walnuts to medium coarseness.
In a large bowl mix pears, onions and walnuts and crumble blue cheese. Mix well.
Preheat oven to 375 degrees. Remove tart shells from freezer and place on baking sheet lined with parchment.
Fill still frozen tart shells generously with mixture.
Bake for 15 to 20 minutes, checking for nice browning of pastry.