Peel and finely chop the onion. Peel and grate or crush the garlic. Rinse the mushrooms and finely slice them. Slice the chicken into bite-sized chunks. Fill the kettle and boil it.
Heat a splash of olive oil in a frying pan. Add the chicken. Season with a pinch of salt and pepper. Cook over a medium-high heat for 5-6 mins till the chicken is golden brown. Lift out of the pan and pop onto a plate.
Add the onion to the pan. Turn the heat down to low. Cook for 5 mins till soft and glossy. Stir every so often. Add the garlic and mushrooms to the onion. Cook for 5 mins till the mushrooms are lightly browned. Stir now and then.
While the veg cook, fill a pan with hot water from the kettle. Bring to the boil. Add the spaghetti. Cover. Simmer for about 8 mins till the spaghetti is cooked but not too soft. It should still have a little bite to it.
Pop the chicken back in the pan with the veg. Add a splash of hot water from the kettle and simmer for around 2-3 mins to cook the chicken right through.
Separate the leaves from the lettuce. Rinse. Dry them in a salad spinner or with kitchen paper. Pop in a bowl. Zest and juice half a lemon. Dress the salad with the lemon juice and a splash of olive oil. Toss to coat.
Drain the spaghetti. Stir the crème fraîche into the chicken and veg. Gently warm for 1-2 mins. Add the spaghetti. Toss to mix. Pop onto two warm plates. Sprinkle with the lemon zest and serve with the salad.