Bring a large pot of water to a boil. Add the sweet potato and cook for 10-15 minutes, or until softened. Drain and mash with 1 tablespoon of milk.
Sprinkle the chicken halves with flour, salt, and pepper, coating the chicken evenly.
Heat a large frying pan over medium heat, add 1 tablespoon of butter, and cook the chicken for 3-4 minutes on each side, or until just cooked through and golden. Set aside.
Add the remaining 1 tablespoon of butter and the mushrooms. Cook for about 5 minutes, or until softened. Add the garlic and stir for 30 seconds.
Meanwhile, bring a small pot of water to a boil. Cook the broccolini for 5 minutes, then drain.
To the frying pan, add the water, chicken stock powder, and cream. Bring to a gentle simmer for about 2 minutes, or until the sauce has thickened slightly. Stir in the Parmesan cheese, return the chicken to the pan, and heat through for 1 minute.
Serve the chicken with sweet potato mash and broccolini, topped with mushroom sauce. Enjoy!