Step 1

Mix the sour cream, mayonnaise, buttermilk, onions, chives, garlic and parsley together in a large bowl. Place the sirloin in the bowl and make sure it is completely covered with the marinade. Cover with plastic wrap and refrigerate for two days.

Step 2

Preheat a grill to high heat. Remove the sirloin from the bowl, wipe off any excess marinade and season with salt and pepper. Grill over high heat, turning once, until the internal temperature on an instant-read thermometer reads 110 degrees, 10 to 15 minutes.