Slow Cooker Pot Roast

3 lb chuck post
3 lbs quartered russet potatoes or baby potatoes
2 lbs quartered peeled carrots
1 tsp onion powder (or 1 sliced onion)
3 teaspoons minced garlic (optional)
½ cup prepared horseradish (do not use the sauce version)
4 cups beef stock
Oil

For the gravy:
2 tbsp flour
2 tbsp butter
2 cups slow cooker cooking liquid