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.