In a large bowl, combine the beef sirloin steak cubes with salt, black pepper, dried thyme, and smoked paprika. Toss to evenly coat the steak pieces with the seasoning.
Heat the olive oil in a large skillet over medium-high heat. Add the seasoned steak bites in batches and sear each side for about 1-2 minutes until browned but not cooked through. Transfer the seared steak bites to the crockpot.
Add the sliced cremini mushrooms to the skillet and sauté for 3-4 minutes until they start to soften. Add half of the minced garlic and cook for an additional 30 seconds until fragrant.
Transfer the mushrooms and garlic to the crockpot with the steak bites.
Cut 3 tablespoons of butter into small pieces and scatter over the steak and mushrooms in the crockpot.
Cover the crockpot and cook on low for 2.5 to 3 hours until the steak bites are tender.
About 10 minutes before serving, melt the remaining 1 tablespoon of butter in a small pan over medium heat. Add the remaining minced garlic and cook for 1 minute until fragrant. Pour the garlic butter over the steak bites in the crockpot and gently stir to combine.
Sprinkle the chopped fresh parsley over the top before serving.