fry the pancetta in a large pan until crisp- about 10 - 15 minutes around medium-high heat.
Remove the pancetta from the pan with a slotted spoon, keeping the grease. Add the onions and fry on medium til deep brown - 10-15 minutes.
Remove the onions from the pan and drain almost all the grease except 1-2 tbsp worth
Cook the tortellini to slightly less than the package directions, drain and add back to the greased pan with the onions and the pancetta. Toss to thouroughly coat the tortellini in the pancetta grease, then fold in the pesto and serve.