Have the butcher cut a whole tenderloin in half so that you can purchase the thick end (they will usually cut the rest into steaks) or buy a "butt beef tenderloin."
Salt and pepper the whole tenderloin thoroughly. Lay the bacon strips out on a clean work surface, touching each other, so they create a rectangular sheet of bacon.
Lay the whole tenderloin across the bacon and carefully wrap the bacon ends over the top. Secure the bacon strips by "sewing" toothpicks through the ends.
Heat a large skillet to medium-high heat. Place the whole tenderloin, toothpick-side-down, in the skillet. Brown the bacon on all sides, turning as needed—10 minutes. *This can be done ahead and the seared tenderloin, can be refrigerated for a 1-2 days if needed.
Place the beef tenderloin in a 5-6 quart Crock-Pot and pour the pan drippings over the top. Place the sliced onion, garlic cloves, and rosemary sprigs around it. Insert an oven-safe meat thermometer in the thickest part of the whole tenderloin, then cover and turn the Crock-Pot on low. A crock-pot beef tenderloin will be perfectly medium-rare when the temperature reaches 130 degrees F—2-4 hrs, depending on weight and thickness.
Once the temperature reaches 100 degrees, mix the flour and sugar together in a medium bowl. Whisk in the balsamic vinegar and mustard into the sugar until there are no clumps. Then whisk in the beef stock. Pour the mixture around the tenderloin, cover, and continue cooking until 130 degrees F is reached.
At 130 degrees F, remove the whole tenderloin and cover with foil for at least 10 minutes. The temperature will continue to rise to 135 degrees as it rests. If needed, allow the balsamic glaze to continue simmering until thick.
To serve, remove the toothpicks and slice the tenderloin into thin rounds. Top with rich balsamic glaze!