Bring a saucepan of lightly salted water to a boil. Add spaghetti and cook until tender, about 8 minutes. Drain.
Meanwhile, heat the oil in a frypan over medium heat. Place the whiting fillet in the centre and add the tomatoes and mushroom. Season with pepper and then squeeze the lemon juice over everything. Place the squeezed lemon half into the frypan.
Cover and cook for about 10 minutes or until fish flakes easily with a fork, turning the fish halfway through. Toss with pasta and cos lettuce. Serve over spaghetti and garnish with Mizithra cheese.