Prep the steak: Pat the steak dry with paper towels. If you want extra tenderness, toss the cubes with baking soda and 1 teaspoon water and let sit 10 minutes, then rinse and pat dry again.
Season generously: In a bowl, combine salt, pepper, onion powder, smoked paprika, and dried thyme.
Sprinkle over the steak and toss to coat evenly.
Load the crockpot: Add the seasoned steak to the slow cooker. Sprinkle in the minced garlic, scatter the butter pieces over the top, and pour in the beef broth, Worcestershire, and soy sauce if using.
Set and cook: Cover and cook on Low for 3.5 to 5 hours, or High for 2 to 3 hours, until the steak is tender but not shredding. Aim for bite-tender, not pulled-beef texture.
Thicken the sauce (optional): Stir cornstarch with 1 tablespoon cold water to make a slurry.
Pour into the crockpot during the last 15 minutes on High, stirring gently until the sauce lightly thickens.
Finish and taste: Stir, taste, and adjust salt and pepper. Add a squeeze of lemon to brighten the butter and garlic flavors.
Garnish and serve: Top with chopped parsley. Serve over mashed potatoes, rice, garlic bread, or roasted vegetables.