Toast sesame seeds in a small skillet over low heat until golden brown.

Combine toasted sesame seeds with water, soy sauce, molasses, vinegar, green onions, garlic powder, and cayenne pepper in a bowl.

Trim excess fat from the beef roast.

Place the beef in a large glass bowl.

Pour the marinade over the beef.

Cover the bowl and refrigerate overnight, turning the meat several times.

Remove the beef from the marinade and pat dry with paper towels.

Brown the pot roast in a large skillet over medium-high heat.

Place the browned pot roast in a slow cooker.

Pour the marinade over the roast in the slow cooker.

Cover and cook on low (190-200°F) for 10 hours, or on high (290-300°F) for 6 hours, or until the beef is very tender.

Remove the beef to a heated platter and keep warm.

Turn the slow cooker heat control to high.

Combine cornstarch with 1/4 cup cold water in a cup and stir to make a slurry.

Stir the cornstarch slurry into the liquid in the slow cooker until well blended.

Cover and simmer for 15 minutes to thicken the gravy.

Carve the meat into 1/4 inch slices.

Serve the sliced meat with the gravy.