Prep & Sear: Pat beef dry, season with salt and pepper. In a skillet over medium‑high heat, melt 1 TB butter and brown beef in batches (2–3 min per side). Transfer to slow cooker.
Make the Roux Base: In the same skillet, melt remaining butter. Sauté onions until soft and golden (6–8 min). Add garlic, cook 30 sec. Stir in flour and whisk 1–2 min until lightly toasted.
Build the Gravy: Gradually whisk in beef broth and water, scraping up fond. Add Worcestershire, mustard, and paprika. Season lightly; simmer 2 min.
Layer & Cook: Place half the bread cubes in slow cooker, top with beef, pour gravy evenly, then add remaining bread. Press down gently. Cook on LOW for 6–8 hr or HIGH for 3–4 hr.
Optional Cheese Finish: During last 15 min on HIGH, sprinkle cheddar over top to melt.
Rest & Garnish: Let stand covered 10 min. Garnish with parsley before serving.