In a small saucepan, heat about 1 Tbsp. olive oil over medium-high heat. Add the diced onion, mustard seeds, and a pinch of salt. Sauté until the onions have softened and become translucent, about 2 minutes. Add the dates, vinegar, and ¼ c. of water and reduce the heat to medium. Cook until the dates have broken down and you have a thick, slightly chunky, jam. If the pan starts to look too dry, add another Tbsp. or 2 of water. Taste and season with salt and coarsely ground black pepper.
Preheat oven to 400°.
Meanwhile, roll-cut the pork tenderloin. Lay the pork on a cutting board and start cutting lengthwise, a ¼-½ in. from the bottom, slowly unrolling the pork as you go. You’ll be left with wide, flat piece of pork. Pound the meat to a uniform thickness, then rub with salt and pepper.
Spread the pork tenderloin with the date and onion “jam”. With the long side of the pork closest to you, roll the pork over itself to form a fairly tight cylinder. If any of the jam tries to sneak out, just push it back in. Wrap the rolled pork with strips of pancetta (mine unrolled itself and was in strips—you can certainly use round slices and overlap them slightly) and secure with a piece of kitchen twine.
In a large ovenproof skillet, heat about 1 Tbsp. olive oil over medium-high heat. Add the pork tenderloin and brown 1-2 minutes per side until the pancetta is golden, about 6 minutes total. Transfer to the oven and roast until the internal temperature is between 135°-140° (it will continue to rise as it rests), 15-18 minutes. Again, if any of the filling tries to escape, push it back in, otherwise it will burn once the pan goes into the oven. Transfer the pork to a cutting board and tent with foil while you make the sauce.
Place the pan you used to roast the pork on the stovetop and add oil, as needed, so you have a thin coating of fat in the bottom of the pan (if there’s too much fat already, skip this and instead drain some off). Remember this pan has been in a very hot oven and the handle is hot—I only remind you because I always seem to forget!! Add the minced onion and sauté until translucent, about 1 minute, scraping up any brown bits crusted to the bottom of the pan as you go. Add the wine, bring to a boil and allow to reduce slightly. Now add the stock, bring to a boil, and reduce to a saucy consistency, 3-5 minutes. Whisk in the Dijon mustard, taste, and season with salt and pepper.
Remove the string from the pork and cut crosswise. Drizzle the sauce over the top or pass at the table.