I Need to Cook Duchess Potatoes but the Oven Cannot go Above 135C or 275F
You are missing an option, which is to let the meat rest before serving and bake the potatoes at the right temperature. Most meats, especially beef, become more tender if they have a chance to rest, as it lets the fibers relax. This is true of slow cooked meat as well. So, your best option is to take the beef out, keep it covered and crank up the oven for your potatoes. If you keep the beef covered or wrapped up in foil it will still be hot enough to serve.