Step 1: Butterfly the flank steak: Position the flank steak lengthwise at the edge of the cutting board. Place one hand on top to hold it flat. Using a sharp, slender knife, cut through the side of the steak to butterfly it (open it up). As you cut, fold open the top to help you see what you’re doing. Open the meat up like a book.
Step 2: Place a large piece of heavy-duty aluminum foil, 24 inches long, on your work surface, shiny side down. (It’s okay to double up two smaller pieces of aluminum foil.) Arrange the strips of bacon on a sheet of aluminum foil, running parallel to an edge and leaving 1-inch spaces between slices. Lay the butterflied flank steak on top of the bacon, with the grain running parallel to the bacon and season with salt, pepper, oregano, and vinegar.
Step 3: Starting at the edge of the meat closest to you, arrange kielbasa, celery, carrot, ham, cheese, and bell pepper on top of the steak in neat parallel rows, alternating colors. Repeat this sequence until all the filling ingredients are used up. Leave the last 2 inches of meat uncovered.
Step 4: Starting at the edge closest to you and using the aluminum foil to help you, roll up the meat with the filling to make a tight roll. Wrap the foil around the meat, twisting the ends of the foil. Using the tip of a paring knife, poke holes in the aluminum foil at 1-inch intervals. Tie the meat in several places with butcher’s string. The matambre can be prepared up to this stage several hours ahead. Store, covered, in the refrigerator.
Step 5: Set up the grill for direct grilling and preheat to medium. When ready to cook, place the roll on the hot grate and grill until cooked through and the meat is very tender, 1 1/2 to 2 hours, turning with tongs every 15 or 20 minutes. If using a charcoal grill, you’ll need to add 24 to 30 fresh coals after 1 hour. To test for doneness, use an instant-read meat thermometer: The internal temperature should be about 180°F. Transfer the matambre to a cutting board and let rest for 10 minutes. Remove the string and foil and cut the roll crosswise into 1/2-inch slices. Serve hot or at room temperature.