Cut steak into 1 to 2-inch pieces.
Melt butter in a skillet over medium-high heat. Quickly sear steak bites for 1 to 2 minutes per side.
Place onion wedges on the bottom of a slow cooker. Add seared steak bites on top.
Whisk Worcestershire sauce, Dijon, maple syrup, garlic, and black pepper together in a small bowl; pour over steak and onions. Pour in beef bone broth.
Cook on Low for 3 hours total. At 1.5 hours; add mushrooms. 15 minutes before the 3-hour mark, remove 2 tablespoons of the cooking liquid and place in a small bowl. Whisk cornstarch into broth; pour back into crockpot. Continue to cook until thickened, about 15 minutes.
Taste and adjust seasoning with salt, pepper, or maple syrup. Garnish with fresh parsley if desired.
