Super critical important detail - DO NOT OVER WORK YOUR MEAT! Overworking the grind is going to leave you with a dense ball of meat worthy of being used for raquet ball... and no one wants that. When mixing, try to use more of a "toss" or "fold" approach as opposed to smashing everything together. You want this to be very loose.
First, add the 1 cup of minced mushrooms and 2 tbsp of chopped chives. Gently mix this in. Once thoroughly mixed, add the 2 tbsp of Meat Church Holy Cow rub and mix. Last, beat the egg in a small bowl, and then pour this into the meat mixture and mix it.
Form 3 oz balls out of the meat, again, keep this loose, we don't want racquet balls here. From 2 lbs of meat, you should get approximately 12. Gently shape each ball into a patty, they will shrink up a fair bit, so you want them to be about twice as wide as the bun you are using, for sliders they will end up being about 1/4" thick.
For best results, these are killer on a charcoal grill, but they would also be awesome on a skillet or in a cast iron pan. If you have a gas grill, that will work fine as well! The idea here is hot and fast. Get your grill up to around 500°. These burgers will take about 7-9 minutes start to finish. Cook on one side for about 4 minutes then flip. Let them remain on the other side until they reach an internal temperature of around 145°.
Place on a nice brioche bun, top with smoked cheddar and red pepper relish and enjoy!