Put the chicken breasts, stock, lemon slice and thyme in a large saucepan. Bring to a very gentle simmer – the water should shiver rather than bubble in the pan. Cover and cook for 15–16 minutes until the chicken is opaque all the way through. Lift the chicken from the liquid with a slotted spoon and transfer to a plate. Remove the lemon slice.
When the chicken is cool enough to handle, shred into large pieces.
Bring the stock to a rapid boil and season with salt and pepper. Add the pasta and cook for 2–3 minutes, adding the shredded chicken for the last minute of the cooking time. Serve immediately with a generous scattering of grated Parmesan.