In a small bowl, combine salt, black pepper, dried thyme, dried rosemary, and smoked paprika. Mix well.
Season the beef sirloin steak cubes evenly with the spice mixture.
Heat olive oil in a large skillet over medium-high heat. Add steak cubes in batches and sear each side for about 1-2 minutes until browned. Transfer seared steak bites to the crockpot.
In the same skillet, add 2 tablespoons of butter and minced garlic. Sauté garlic for about 1 minute until fragrant, then add the cremini mushrooms. Cook mushrooms for 3-4 minutes until slightly softened.
Transfer the garlic butter mushrooms to the crockpot with the steak bites.
Add the remaining 2 tablespoons of butter on top of the steak and mushrooms in the crockpot.
Cover and cook on low for 3 hours 30 minutes until the steak is tender and flavors meld together.
After cooking, stir gently to combine the garlic butter with the steak bites and mushrooms.
Sprinkle chopped fresh parsley over the dish before serving.