Preheat a 9 inch cast iron skillet over medium high heat. Add the bacon pieces and cook until the fat is rendered and the bacon pieces are crisp. Remove the bacon pieces to a paper towel to drain and leave the bacon fat in the pan.
Add in the butter and the sliced red onions. Cook for about 20 minutes over medium heat, stirring occasionally, until the onions have darkened considerably in color. Reduce the heat to low and stir in the balsamic vinegar. Remove the onions from the pan and set aside.
Place the pork tenderloin on a large cutting board. Remove any excess fat or silverskin from the exterior of the tenderloin.
Butterfly open the pork tenderloin by using long strokes of your knife along the side of the tenderloin about 1/2 inch above the cutting board. Keep the slices parallel to the cutting board and roll open the tenderloin while slicing.
Spread the softened cream cheese on top of the butterflied tenderloin. Sprinkle with the shredded manchego cheese, caramelized onions, baby spinach leaves, and bacon pieces.
Starting with the edge of the pork tenderloin that was the inside (before slicing open) begin tightly rolling the tenderloin back up. It's OK if there are a few casualties that fall out the sides while rolling.
Tie the roll together with butcher's twine every inch to inch and a half until the roll is nice and bound. Season the pork tenderloin very liberally with Grill Mates Mesquite seasoning.
Grill over indirect, medium high heat (about 375 degrees F) for approximately 20-25 minutes or until the internal temperature of the tenderloin is 145 degrees F in the thickest part of the meat. Turn the pork tenderloin every 5 minutes while cooking.
Allow the tenderloin to rest at least 5 minutes before removing the butcher's twine, slicing the tenderloin into 1 inch thick rounds, and serving.