2) Heat a lightly oiled griddle pan over medium heat. Brush onions and mushrooms with oil and season well with salt and freshly ground black pepper.
3) When griddle pan is hot, add onions cut side down and cook for about 10 minutes, until softened. Transfer to a warm serving plate.
4) Add mushrooms to griddle pan and cook for about 10 minutes, until charred and soft. Transfer to serving plate.
5) Serve immediately with sour cream sauce.