Step 1 — Load the vegetables. Put the potatoes and carrots in the bottom of the slow cooker and scatter the onion wedges and garlic over them. This bed of vegetables lifts the roast and flavors everything from below.
Step 2 — Sear the roast (optional, for a deeper crust). Pat the roast dry, rub it all over with the salt, pepper, garlic powder, onion powder, and paprika, and sear it in hot oil in a heavy skillet, 4–5 minutes per side, until a deep brown crust forms. Lay it on top of the vegetables and pour the skillet juices over it. Skipping the sear? Season the roast the same way and lay it straight on the vegetables — the roast still turns out tender and savory; the crust is flavor bonus, not a requirement.
Step 3 — Add the liquid. Whisk the broth, Worcestershire, and tomato paste with the thyme and rosemary, and pour it around (not over) the roast. Tuck in the bay leaves. The liquid should come partway up the vegetables, not cover the roast — pot roast braises, it doesn't boil.
Step 4 — Cook low and slow. Cover and cook on LOW for 8–9 hours or HIGH for 5–6 hours, until the roast is fork-tender and the vegetables are soft. Keep the lid closed — every peek costs heat and time.
Step 5 — Rest the roast. Lift the roast onto a cutting board, tent it loosely with foil, and rest it 15 minutes. This keeps the juices in the meat instead of all over the board. Rest the vegetables in the cooker with the lid on to stay warm.
Step 6 — Make the gravy. Strain 2 cups of the cooking liquid into a saucepan (a fat separator makes this effortless; otherwise skim the surface with a spoon). Bring it to a gentle boil, whisk in the cornstarch slurry, and simmer 1–2 minutes until glossy and thick enough to coat a spoon. Taste — it rarely needs salt, but a grind of pepper is nice. (In-cooker shortcut: whisk the slurry straight into the cooker juices and cook 20 minutes on HIGH. The saucepan version is smoother and worth it on Sundays.)
Step 7 — Slice and serve. Slice the roast against the grain (or pull it into chunks for the rustic version), arrange it over the vegetables, and spoon the gravy generously over everything. Parsley on top, extra gravy at the table.