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 6-8 hours (or until center reaches 135-140 degrees F to be medium, still pink and tender inside - timing will vary depending on size of roast).
When done turn slow cooker off, remove roast and cover it with aluminum foil to rest for 15 minutes while you make the gravy.
To Make Gravy - Whisk heavy cream with cornstarch in a bowl until smooth. Pour into the pot and mix with the drippings in your Crockpot so the gravy thickens. Leave lid on until roast has rested, you've sliced it and is served together.