Place the sirloin cubes directly in the slow cooker.
Add garlic, paprika, onion powder, salt, and pepper.
In a small bowl, whisk together melted butter, olive oil, soy sauce, and Worcestershire.
Pour over the steak.
Cook
Cover and cook on LOW for 3–4 hours or HIGH for 1–2 hours, until the sirloin is tender but still meaty. The temperature should be around 145°F (63°C) = Medium, 160°F (71°C) = Medium-well or 165°F (74°C) = Well done
Stir in the Boursin cheese during the last 15 minutes of cooking.
If you want more of a sauce, add up to 2 extra tbsp broth while melting in the cheese.