Heat a large skillet over medium-high heat. Season roast with kosher salt and 1 teaspoon black pepper. Add vegetable oil to hot skillet. Add roast and sear each side, about 2 mintues per side.
Meanwhile, add soup mix, beef broth, worcestershire sauce, Dijon mustard, butter and remaining 1/2 teaspoon pepper. Whisk well. Once done searing the roast, add it to the slow cooker. Flip so both sides get coated in the sauce.
Cook on low for 6 to 8 hours or high for 4 to 5 hours.
After time has elapsed, remove the roast from the slow cooker to a plate. In a small bowl, whisk together the cornstarch and cold water. Add to slow cooker and whisk well. Return roast to slow cooker and cook another 10 minutes, until sauce is thickened and glossy.