Step 1 — Sear the beef (optional, for deeper flavor). Pat the beef dry, toss it with the flour and a pinch of the salt and pepper, and sear in hot oil in two batches, 2–3 minutes per side. Move it to the slow cooker and deglaze the skillet with a splash of the broth, scraping up the browned bits — pour every drop over the beef. Skipping the sear? Put the raw beef straight into the cooker, seasoned as below; it still turns out tender and delicious, with a slightly lighter gravy.

Step 2 — Build the gravy base. Add the onion, mushrooms (if using), and garlic over the beef. Whisk the broth, Worcestershire, soy sauce, tomato paste, garlic powder, onion powder, thyme, salt, and pepper, and pour it over everything.

Step 3 — Cook low and slow. Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours, until the beef is very tender and the gravy smells rich and savory. Try a piece at the early end of the range — when it gives easily, you are there.

Step 4 — Thicken the gravy. Stir the cornstarch and cold water into a smooth slurry, stir it into the hot gravy, cover, and cook on HIGH for 15–20 minutes until the gravy turns glossy and coats a spoon. Too thick? Whisk in broth a splash at a time. Too thin still? A second small slurry fixes it.

Step 5 — Finish and serve. Taste and adjust the salt and pepper — and for a richer finish, stir in 2 tablespoons of sour cream or a splash of heavy cream off the heat. Spoon over mashed potatoes, rice, or egg noodles, and scatter with parsley.