- Cut beef sirloin tips into 1 1/2-inch cubes.  
- Halve the cremini mushrooms.  
- Cut the red onion into 1 1/2-inch chunks, keeping the layers stacked together to stay secure on the skewer.  
- Mince garlic and grate half of a medium onion.  
- In a large bowl, mix Greek yogurt, minced garlic, grated onion, ButcherBox BBQ seasoning, black pepper, and kosher salt until smooth.  
- Add the cubed beef to the marinade and toss to coat well. Cover and refrigerate for at least 2 hours, ideally overnight for maximum flavor.  
- Soak wooden skewers in water for at least 30 minutes before grilling to prevent burning.