Combine the first seven liquid ingredients for gravy in a small bowl with the cornstarch mixture. Lay sliced onion on the bottom of slow cooker. Add the vegetables and add the meat on top. Pour the contents of the bowl with the liquids on top. (I do not add any further spices, however you may add salt and pepper). Leave on High for one hour and medium for the next 4 hours or more. Transfer roast to a cutting board; thinly slice against the grain. Place vegetables in a serving dish and pour gravy through a sieve over all.