Mix the ground chuck with baking soda, Worcestershire sauce, and black pepper. Handle gently to avoid overmixing. Shape into ½ lb patties using a scale for consistency. Place on patty paper and refrigerate until ready to cook.
Heat olive oil in a non-stick pan over medium heat. Add mushrooms, season with salt and pepper. Cook until water is released and evaporated, leaving flavorful sautéed mushrooms. Set aside.
In another pan, melt butter and add sliced onions with salt and pepper. Cook slowly over medium-low heat until onions soften, release moisture, and caramelize into a jam-like spread (about 30 minutes).
Preheat a cast iron pan over medium-high heat. Place patties onto the hot surface and cook undisturbed for 3½ minutes. Season with salt. Flip, season the other side, and cook another 3½ minutes.
Top each patty with sautéed mushrooms and a slice of Gruyere cheese. Cover with a lid to melt the cheese. Remove from heat.
Spread mayonnaise on the bottom bun. Place the cheesy mushroom burger on top. Spread caramelized onion jam on the top bun and cover the burger. Serve hot.
