Season & Dredge: Pat the cubed steak dry. Mix salt, pepper, garlic powder, paprika, and flour. Lightly coat each piece of steak in the seasoned flour.
Sear the Steak: Heat oil/butter in a skillet over medium-high heat. Sear steak 1–2 minutes per side until golden. Work in batches to avoid steaming.
Layer the Crockpot: Add sliced onions to the bottom of the slow cooker. Place the seared steak on top. Add mushrooms if using.
Make the Gravy: Whisk beef broth, gravy mix, Worcestershire sauce, and thyme until smooth. Pour evenly over the steak.
Slow Cook: Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours until the meat is fork-tender and the gravy thickens.
Finish & Serve: Taste and adjust seasoning. Let rest 10 minutes before serving for the gravy to settle.