Season the sirloin steak cubes with salt, black pepper, dried thyme, and dried rosemary. Toss to coat evenly.
Heat the olive oil in a large skillet over medium-high heat. Add the steak cubes in batches and sear for 1-2 minutes on each side until browned but not cooked through. Transfer the seared steak bites to the crockpot.
In the same skillet, add 2 tablespoons of butter. Once melted, add the minced garlic and sauté for 1 minute until fragrant.
Add the halved cremini mushrooms to the skillet and cook for 3-4 minutes until they start to soften.
Transfer the garlic and mushroom mixture to the crockpot with the steak bites.
Add the beef broth and remaining 2 tablespoons of butter to the crockpot. Stir gently to combine all ingredients.
Cover and cook on low for 3 to 3.5 hours until the steak bites are tender and the mushrooms are fully cooked.
Once cooked, sprinkle the chopped fresh parsley over the steak bites and mushrooms. Stir gently to combine.
Serve immediately, spooning the garlic butter sauce over the steak bites and mushrooms.