Heat oven to 200C/180C fan/gas 6. Season the chicken inside and out, then stuff the cavity with 1 of the lemons and the thyme. Sit in a roomy roasting tin, drizzle with olive oil and roast for 40 mins.
Remove the tin from the oven, then scatter the gluten free ciabatta pieces around the chicken and ruffle over the prosciutto. Add the remaining lemon halves to the tin. Roast everything together for 40 mins more until the gluten free croutons and prosciutto are crisp. Remove from the oven, transfer the chicken to a plate and leave to cool. Remove the croutons and prosciutto, and set aside.
While the chicken is cooling, cook the green beans in a pan of boiling water for 5 mins. Drain, cool under cold water, then tip into a big bowl with the parsley.
To make the dressing, squash the roasted lemon in the roasting tin with a potato masher. Stir in the olive oil and lemon juice with some seasoning – scrape the bottom of the tin to get all the sticky chicken juices into your dressing. Stir in any resting juices from the chicken and set aside.
When the chicken is cool enough to handle, strip the meat off the bones. Just before serving, add the dressing, rocket, chicken, gluten free croutons, prosciutto and lettuce to the green bean mix in the bowl, toss together and scatter over a large serving platter.