Heat a skillet over medium heat. Cook the bacon until crisp and remove to paper towels reserving the fat in the skillet. Add the mushrooms, spinach and garlic to the pan and sauté until the mushrooms have softened and given off their liquid. Transfer the mixture to a food processor with the bacon, breadcrumbs, parmesan, salt and pepper and pulse 5-10 times until the mixture is chunky but not pureed.
Butterfly each pork tenderloin lengthwise and place between two pieces of plastic wrap. With a rolling pin or a mallet, flatten each tenderloin until they are each about 1/3-inch thick. Spread half of the mushroom mixture on the inside of each pork tenderloin. Roll up each tenderloin like a jelly roll and secure with toothpicks. Season the outside with salt and pepper.
Ready a grill for indirect cooking (that means your coals are pushed to one side). Cook away from the coals for 15-20 minutes or until the interior of the pork tenderloin reaches 140 degrees. Let stand for 10 minutes before slicing.
Mix the parsley, lemon zest and olive oil together in a small bowl and serve on top of the sliced pork.