Rinse pot roast and pat dry.

Heat the oil in a skillet over medium-high heat; brown meat on all sides; transfer to a slow cooker, fat side down. Lay garlic slices on top of beef; sprinkle lightly with salt and pepper.

Cover and cook on low heat setting for 4 to 5 hours, depending on the shape of the roast. Check for tenderness, then transfer roast to a cutting board.

Skim all visible fat from the top of the slow cooker juices, then transfer liquid to a saucepan over medium heat. Dissolve cornstarch in water or milk; add to saucepan and cook, stirring until thickened. Salt and pepper to taste. Serve roast with gravy drizzled over the top.