Preheat the oven to 180C/350F/Gas 4.
For the lemon and garlic-roasted chicken, cut two slashes into the chicken breast. Tuck the garlic and cheese into the pocket under the mini-fillet, cutting to make it larger if necessary.
Place the thyme leaves, lemon juice and zest and olive oil into a bowl, season with salt and freshly ground black pepper and mix well.
Place the chicken into the bowl and leave to marinate for ten minutes.
Heat a griddle pan and griddle the chicken for 3-4 minutes on each side. Transfer to a baking sheet and roast in the oven for 12-14 minutes, or until completely cooked through.
Meanwhile, for the cheesy butternut mash, place the squash, garlic and milk into a large pan. Bring to the boil, reduce the heat and simmer for 10-12 minutes, or until the squash is tender. Mash with a potato masher, season well with salt and freshly ground black pepper and stir in the grated cheese.
To serve, place the mash onto a plate, top with the chicken and pour over the chicken cooking juices.