Mix the seasoning salt, garlic powder, Italian seasoning, onion powder, and black pepper in a small bowl. Rub the blend all over the roast until it's fully coated.
Heat oil in a large skillet over high heat. Once hot, sear the roast on all sides until nicely browned—this locks in flavor. Transfer the roast to a 4–6 quart slow cooker.
In the same pan, reduce the heat to medium and add the chopped onion (more oil if necessary). Sauté until soft and translucent. Then stir in the water and bouillon, tomato paste, balsamic vinegar, Worcestershire sauce, and honey.
Whisk together the cornstarch and water in a small bowl, then pour it into the pan. Let everything simmer for 2–3 minutes, scraping up any browned bits from the bottom for extra flavor.
Add the sliced carrots around the roast in the slow cooker. (If you’re using potatoes, you can add them now or wait 2–3 hours for firmer results.) Pour the sauce mixture evenly over everything.
Cover and cook on low for 8 hours, or high for 4, until the roast is fork-tender and the vegetables are cooked through.
Once done, remove the roast and slice or shred. If desired, thicken some more with another round a cornstarch slurry. You can also transfer the juices to a saucepan through a strainer and thicken further with a cornstarch slurry if you want a smoother gravy.