Heat oil in a large skillet. Sauté the onions for about 15 minutes or until edges are brown. Season to taste. Add mushrooms and cook until all the liquid has been released and evaporated and the mushrooms are very well browned, about 20 minutes. Season with salt and pepper to taste. Transfer to a container.

Combine with mayonnaise, salt, pepper, lemon juice, cocoa, and sugar. Blend with an immersion blender until very smooth. Add water until you reach desired consistency. Store in the fridge for up to one to two weeks.