Heat the oven. Preheat the oven to 350°F. 

Caramelize the onions. Melt the butter in a medium pan over medium-low heat.  Add the onions, stir and let them sit for about 20 minutes so they can begin to caramelize.  Stir every 10 minutes or so, ensuring they don't turn too dark.  Once the fond, which is the dark bits that form on the bottom of the pan, starts to form, deglaze the pan by adding the sherry and scraping the pan with a silicone spatula.  Scrape as much of the fond back into the onions as you can.  

Cook the pancetta. While the onions are caramelizing, add the pancetta to a small skillet over medium heat.  Cook for 5-6 minutes or until the pancetta is browned and crisp.  Remove from the pan and drain on a plate lined with paper towels.  

Prepare the crust. Lightly coat 4 mini quiche (or tart) pans or 24 mini muffin cups with oil. Unroll one of the pie crusts on a rolling mat or clean counter.  Roll out slightly with a rolling pin. If using quiche pans, cut the crust in half and press into half into each quiche pan. Cut the edges s so they are even with the rim of the pan.  Repeat with second pie crust.  You will have leftover pie crust.  

If using mini muffin pans, cut 24 2 2/1 inch circles (a biscuit cutter or small glass works well for this).  You will need to scrape the pieces and re-roll one time.  Again, you will have some leftover crust.  Press the circles into the muffin cups.  

When the onions are done cooking, divide them evenly between whichever type of pans you are using. Then sprinkle each tart with a little pancetta and some blue cheese.  Bake for 16 - 20 minutes or until crust starts to turn golden brown.  

Remove from oven and let cool slightly.  If making mini tarts, remove them from the pan after a few minutes.  Serve warm.