Prepare the sugar cookie mix according to package directions and cool.
Preheat the oven to 350 degrees F. Line a rimmed baking sheet with aluminum foil.
Break the sugar cookies into large chunks. In the work bowl of a food processor, place the cookies and breadcrumbs together and pulse until crumbly. Remove 2 cups of the crumbs, and place on a separate baking sheet.
In a small bowl, stir together the mustard, and some salt and pepper. Coat the tenderloins with the mustard mixture, and then roll in the cookie crumbs mixture. Place on the prepared baking sheet. Bake until a meat thermometer inserted in the tenderloin registers 155 degrees F, 25 to 35 minutes. Remove from the oven, and let stand 10 minutes before slicing.
To serve, thinly slice the tenderloin crosswise. Place 1 to 2 teaspoons cranberry sauce on the bottom half of each bun. Top with the sliced pork, and mayonnaise, onions and lettuce, if desired. Then cover with the bun top. Serve warm or at room temperature.
Place the remaining crumbs in a re-sealable plastic bag, and freeze for another use. The crumbs can be frozen for up to 3 months.