Pat the tri tip dry with paper towels. Combine the salt, pepper, garlic powder, onion powder, and smoked paprika, then rub over all surfaces including the thin ends.

Heat the oil in a cast iron skillet over medium-high until shimmering. Sear the roast fat-cap-down for 3–4 minutes without moving it. Flip and sear the second side 3–4 minutes, then sear each thin edge about 1 minute using tongs.

Spread the sliced onion and smashed garlic across the bottom of the slow cooker. Set the seared roast on top.

Return the skillet to the heat, add the beef broth, and scrape up all browned bits. Whisk in the Worcestershire, soy sauce, and tomato paste, then pour around the roast rather than over it. Add the rosemary and bay leaves.

Cover and cook on LOW 4–5 hours for sliceable tri tip (145–155°F internal), or 7–8 hours for shreddable tri tip (200–205°F internal).

Transfer the roast to a cutting board, tent with foil, and rest 15 minutes.

Cut the roast in half where the two grain directions meet, then slice each half against its own grain about ¼ inch thick.

Strain the cooking liquid into a saucepan, discarding the bay leaves and rosemary stems. Bring to a simmer. Whisk the cornstarch into the cold water until smooth, stream into the liquid while whisking, and simmer 2–3 minutes until it coats a spoon. Season to taste and serve over the beef.