Heat oil in a heavy skillet and cook pork roast until browned on all sides, turning occasionally. This should take about 10 minutes. While that's cooking, cut each acorn squash into 8 wedges and remove seeds.
Do not peel. Cut sweet potatoes into chunks. Place squash and sweet potatoes in 4-6 quart slow cooker. Top with browned pork roast.
In a small bowl, mix together apple butter, horseradish, cornstarch, allspice, and pepper. Pour into slow cooker.
Cover crockpot and cook on low for 7-9 hours until pork and vegetables are tender.