To make onions

1. In a medium cast iron pan over medium-high heat, melt butter and oil. Add onions and a pinch of salt; stir to coat.
2. Reduce heat to low and cook, stirring every few minutes, until the onions are softened and golden brown, at least 20 minutes.
3. Remove from heat, add whiskey, stir to coat, and return to the rangetop. Over medium heat, cook just until the alcohol smell is cooked off; set aside.

To make mushrooms

1. In a medium cast iron pan over medium-high heat, melt butter and oil. When oil shimmers, add mushrooms, thyme, and garlic; stir to coat in oil, season with salt and pepper, and cook undisturbed at least 5 minutes.
2. Stirring occasionally, cook until the mushrooms have released excess water and turn golden brown, about 5 to 10 minutes more. Remove from heat and set aside.

To make burgers

1. Heat griddle to 375°F. In a large bowl, mix bison and salt until well combined.
2. Shape into eight ¼-inch patties; brush with vegetable oil. 3. Cook patties until browned on first side, about 1–2 minutes. Flip and cook to desired doneness, about 1–2 minutes more.
3. Serve sliders on buns topped with mayonnaise, mushrooms, onions, and arugula.