For the fricassée, heat the butter in a frying pan, add the onion and garlic and fry for 3-4 minutes, or until softened but not coloured. Add the mushrooms and fry for 2-3 minutes, or until golden-brown.
Add a splash of white wine, then reduce the heat and simmer for 2-3 minutes. Stir in the cream, season to taste with salt and freshly ground black pepper and simmer until the sauce has reduced by half. Stir in the nettles and add a squeeze of lemon juice, to taste.
For the wilted nettles, heat the olive oil in a frying pan and add the nettles. Season with salt and freshly ground black pepper, and leave to wilt in the heat for 1-2 minutes.
To serve, pile the wilted nettles onto a serving plate, place the toast on top and then pour over the mushroom fricassée.