Juice 3 lime halves and pour into a medium ceramic or glass bowl. Add 2 tablespoons of olive oil and season with salt and pepper. Whisk until well combined. Add mushrooms and stir to coat. Cover and set aside 30 minutes to marinade.
Cut the remaining lime halves in thirds. Cut salmon into 3cm cubes. Thread mushrooms, salmon and green onions alternately onto 8 skewers. Press 1 lime wedge onto each end. Brush both sides of each skewer with remaining oil and season well.
Preheat frying pan or barbecue plate on medium-high. Cook skewers, 6 minutes, turning to cook all sides. Remove to a tray and allow to stand 5 minutes.
Meanwhile, combine pesto and crème fraiche together, serve with salmon & mushroom kebabs.