Place onion, leek, and garlic in bottom of slow cooker.
Season roast with salt and pepper on all sides.
Heat skillet over high heat and add butter and oil. Once the skillet is very hot, carefully add the beef roast. Sear on all sides for about 30 seconds until you have a nice crust. Transfer seared roast to your slow cooker.
Cover beef in tomato paste
Carefully pour red wine and beef broth over the beef roast.
Cook in your slow cooker on the low setting for 7-8 hours or on high setting for 4 hours.
When done, transfer beef roast to a plate. To thicken the sauce, pour 1 cup of the sauce into a bowl and whisk in a tablespoon of cornstarch. Pour sauce back into the slow cooker and stir to combine.
Pour sauce over roast and serve with your favorite side dishes. Enjoy!
