Cook onions: Heat oil in a large Dutch oven over medium-high. Add onion, mushrooms, garlic, rosemary, Italian seasoning, salt, and pepper.
Add broth, tomatoes, and potatoes: Cook, stirring often, until onion softens slightly, about 4 minutes. Add broth, tomatoes, and potatoes.
Add roast and frozen vegtables: Cover and bring to a boil over high. Uncover and add pot roast with gravy and frozen vegetables; cook over high until flavors meld, about 10 minutes. Discard rosemary sprig. Season with salt to taste. Garnish with thyme, if desired.
