Melt the butter in a wide heavy pan over medium-high heat. Add the diced onion and cook for 3 to 4 minutes until softened and beginning to turn translucent. Add the finely chopped mushrooms to the pan. Season generously with salt and black pepper. Cook over medium-high heat, stirring occasionally, until the mushrooms have released all their moisture, the liquid has evaporated completely, and the mushrooms are concentrated, slightly caramelised and deeply fragrant. This takes approximately 10 to 12 minutes and requires patience. Do not rush it by using higher heat or the mushrooms will steam rather than sauté. The concentrated, caramelised mushroom mixture is the flavour foundation of the entire dish.