In a large microwave-safe bowl, combine potatoes and 1 cup stock. Microwave on high, covered, until just softened, 12-15 minutes. Transfer mixture to a 6-qt. slow cooker. Combine bread and remaining stock in a large bowl; let stand until liquid is absorbed. , Meanwhile, pulse mushrooms, onion, carrot, celery and garlic in a food processor until finely chopped. Add vegetable mixture, eggs, beef, pork, Worcestershire sauce, 1-1/4 teaspoons salt and 3/4 teaspoon pepper to bread mixture; mix lightly but thoroughly. Place meat mixture on an 18x12-in. piece of heavy-duty foil; shape into a 10x6-in. oval loaf. Lifting with foil, place in slow cooker on top of potatoes; press foil up sides and over edges of slow cooker, creating a bowl to contain meat loaf juices. Combine glaze ingredients; spread over loaf., Cook, covered, on low until a thermometer reads 160°, 4-5 hours. Using a turkey baster, remove and discard liquid contained in foil; lifting with foil, remove meat loaf to a platter (or carefully remove meat loaf using foil, draining liquid into a small bowl). Let stand 10 minutes before cutting., Drain potatoes, reserving cooking liquid; transfer potatoes to a large bowl. Mash, gradually adding butter, remaining 3/4 tsp. salt and 1/4 tsp. pepper and enough reserved cooking liquid to reach desired consistency. Serve with meat loaf.