Line bottom of slow cooker with sliced onions. Add vegetables if using them. Place roast on top of bed of onions.
In a bowl, whisk together dry mixes, water and gravy. Pour entire mixture over roast and cook on low 8 hours.
To Make Gravy
Remove 1.5 cups of the liquid from the slow cooker when roast is done and put into a small saucepan. Place 1 teaspoons of cornstarch into a small bowl. Add 1/4 cup of the reserved liquid from the saucepan and mix with cornstarch until smooth. Add this mixture back into the remaining juice in the saucepan and cook over low heat until thickened.
Serve with boiled potatoes, gravy and crusty bread.