For the burger mixture: 

1. Place venison and pork mixture into a large bowl. Add Worcestershire, fish sauce, garlic, salt and pepper then mix together loosely with hands. Once thoroughly mixed, grate frozen butter into the bowl and briefly fold into the meat. 
2. Divide mixture into 4-6 portions depending on the size of patty you want (I prefer a double stack of thin patties so typically make 5 or 6). Roll the portions into meatballs, cover with plastic, and place in the fridge until ready to cook. 

Making the smash burgers: 

1. In a cast iron skillet, place thinly slice onions into rings then fry over medium-low heat in small amount of butter & olive oil, remove from pan when edges brown. 
2. Bring burner up to medium-high heat and add a small drizzle of oil. Add 1-2 meatballs depending on size of pan. 
3. Place a piece of parchment paper underneath a burger press or heavy-bottomed steel pot and push down firmly to “smash” the burgers into thin patties. Make sure to apply even pressure so that the patties are a consistent thickness. 
4. Cook 1.5 – 2.5 minutes until the edges caramelize. If desired, season patty additionally with S&P 
5. Flip burger and cook for an additional 1.5 – 2.5 minutes. When the patty is less than 1 minute from done, add 1 slice of cheese if desired. Remove from pan
6. After all burgers have been cooked, quickly re-fire the onions to crisp them in the pan.
7. Assemble the burgers with lettuce, tomato and burger sauce immediately. Enjoy!