Pat the beef roast dry and place in the bottom of the crock pot.Add in the beef boulion, water, oregano and basil and stir it all together well.
Turn the slow cooker on low and while it begins to heat up, dice your onion and celery and add it to the slow cooker. At this point you will want to add your baby carrots to the slow cooker as well.
Keep the slow cooker temperature on low for three hours. Once three hours has passed, wash, peel and halve your potatoes and add them to the slow cooker. (If you are unable to do this step at three hours, do this step when you add the onion, celery and baby carrots.)
Once you have added the potatoes, let it all simmer together on the low heat setting for another 5-7 hours, for a total of 8-10 hours of cooking.
After the 8-10 hours of cooking has passed, shut the slow cooker off and begin straining out the beef and vegetables from the broth that has been created.
Once you have strained the broth from the beef and vegetables, pour the broth into a 2-quart saucepan and place over medium heat. While the broth is beginning to heat up, mix the milk and cornstarch together in a small container to form a pourable paste.
Bring the broth to a boil and once it's to a boil, slowly pour in the cornstarch paste. Be sure to constantly stir the broth while pouring in the paste, otherwise the gravy will be clumpy.
After the gravy is complete, you have yourself a nice dinner complete with roast beef, potatoes and veggies. Enjoy!