Tip cream, milk, garlic and horseradish into a large saucepan, season, then bring to the boil. Reduce the heat, add the potatoes, stir to stop them sticking together, then cook for 10 mins until tender.
Pour the potatoes and horseradish cream into an ovenproof dish. You can do this up to a day ahead. Cover, store in the fridge, then bring back to room temp an hour before you want to bake.
Bake for the 30 mins at 190C/ 170C fan/ gas 5, (if you're cooking Roast beef & carrots with easy gravy, you can pop it on the shelf below the beef for the last 30 minutes of the beef's cooking time); then increase the temp to 220C/200C fan/ gas 7 and cook for another 30-35 mins. (
Add the muffin tin for the Yorkshires when you turn the oven up.)
