Pour olive oil into a medium skillet and place over low heat. Add onions and cook gently until they are translucent, 3–4 minutes. Add mushrooms and cook until they start to soften, about 3–4 minutes. Pour in heavy cream and cook, stirring frequently, for another 3 minutes, until the mixture thickens. Season with sea salt and black pepper.

Toast the bread slices to your liking. Top with a generous scoop of warm mushrooms, sprinkle with a little more sea salt, and top with parsley.