Place the potatoes, carrots and onions in a 6-qt. slow cooker. In a large skillet, brown beef in oil, a single layer at a time; place over the vegetables. Sprinkle with flour. Dissolve bouillon in boiling water. Stir in the vinegar, ketchup, horseradish, mustard and sugar; pour over meat and vegetables. Cover and cook on high for 5 hours. Add the peas, corn and mushrooms. Cover and cook on high for 45 minutes longer or until meat and vegetables are tender.