Place the pork tenderloin in a Dutch oven with about an inch of water or chicken broth. Slice an onion and add to the Dutch oven. Salt and pepper to taste. Bake in a preheated oven for about 1 1/2 hours or until the center of the tenderloin is no longer pink. Remove the tenderloin from the Dutch oven and shred. Discard the water and onion. Place the shredded meat back into the Dutch oven.
Mix together the ketchup, Worcestershire sauce, brown sugar, vinegar, cumin, and paprika until well combined. Pour over the shredded meat and stir until the meat is completely coated.
Reduce the temperature of the oven to 300 and bake the pork for another hour.
While the pork is finishing baking make the onion strings. Cut the onion into thin slices. Working in batches dredge the slices of onion with the flour. Shaking the excess flour off of the onion before frying.
In a skillet add enough oil so that you have about an inch of oil in the skillet. Test the oil with a drop of water if it sizzles the oil is ready to start. Add the onions just enough to fill the pan so that they are not crowded. Fry the onion until brown. Once they are browned place on paper towel.
To assemble the tacos place the bbq pork on the taco shell. Top with the onion strings and then with the Monterey jack cheese