Slice yellow onions until paper thin. On medium heat, melt 2 tbsp butter in saucepan. Add bourbon and brown sugar. Once combined, reduce heat to low and add yellow onions. Saute for about 30 minutes until onions are translucent.
Cook bacon until almost crispy. Remove from heat and set aside.
Salt Burger on both sides and cook on high heat to order (approximately 4-5 minutes per side for Medium-Rare).
When Burger is nearly cooked, melt remaining butter in a pan and place brioche bun (cut in half) inside down to toast.
Build Your Boursin Burger: bottom bun, onions, Burger, bacon, Boursin cheese (apply to inside of top bun), top bun.