Instructions

1. Fold bone marrow fat into butter and set aside.
2. In a mixing bowl, mix ground beef and ground mushrooms, form in patties.
3. In a separate mixing bowl, mix sliced yellow onions and yellow mustard, once evenly covered, cryovac onions and sous vide for 45 minutes at 88.6 C.
4. Remove onions from bag and strain off any excess liquid.
5. Heat a medium frying pan over medium high heat and place onions and 1 tbsp canola oil and 1 tsp black pepper into pan.
6. Cook onions, stirring occasionally for 3 minutes, until onions are softened throughout.
7. Place burgers on a flat baking sheet and season liberally with kosher salt and black pepper on both sides.
8. Heat a griddle over medium high heat and place burgers on hot griddle, giving light press on the top of the burger just enough to ensure the surface is touching the griddle.
9. Bone marrow the inside of burger buns and place on griddle.
10. Using the palm of your hand or side of your knife, smash 2 garlic cloves.
11. Add smashed garlic and rosemary to griddle in between the burger patties and let cook for 2 minutes.
12. Once burgers are browned on the bottom side, flip burger patties and top with American cheese. Remove from griddle and place on baking sheet to rest.
13. Place burger buns back on griddle for 30 seconds, to soak up all the cooking juices and flavor the buns.
14. Spread ketchup, mustard and mayonnaise on top of buns.
15. Layer cooked onions on ketchup.
16. Place cooked burger patties on top of onions and finish with bread and butter pickles and the top bun.
17. Serve