Return both halves of the roast to the pot along with broth, tomato sauce, rosemary, thyme, and salt and pepper.
Stir together. Add the lid and set the timer for 60 minutes. Then, let it naturally release for 20 minutes. Remove meat and set aside.
Combine corn starch to the broth and stir. Slowly add the slurry to the Instant Pot and stir until the it thickens.