Heat a skillet on medium high heat, melt butter. Then season all sides of roast with salt and pepper and brown on all sides in the skillet.
Place potatoes and carrots inside slow cooker, sprinkle onion soup mix on top. Put roast on top of that.
In a dish whisk together garlic, red wine vinegar, worcestershire, horseradish sauce and broth. Pour this on top of everything.
Close and cook on low for 7-8 hours (timing depends on how well done you want it to be inside.
When done whisk heavy cream with cornstarch until smooth. Remove roast and let that rest. Pour cream into liquid in pot and mix so gravy thickens. Leave lid on until roast has rested, you've sliced it and is served together.