Cut the onion in half and slice finely. Wipe the mushrooms, discard the stalks and slice finely.
Melt the butter with the olive oil in a non-stick fry pan, and gently cook the onion for 5 minutes. Add the mushrooms and cook for 5 minutes. Add white wine, sea salt, pepper and paprika, and bring to the boil, stirring. Reduce the heat and simmer for 5 minutes.
Heat a cast-iron gril