Place the chicken stock into a small saucepan, warm on a medium to high heat and bring to the boil. Add in the potato and cauliflower, cover with a lid and cook for 6-8 minutes, or until the vegetables are soft.Drain any excess stock from the vegetables into a small bowl and keep it to use later. Place the cauliflower and potato into a small blender, along with the ricotta and a pinch of salt and pepper. Blend until smooth, set aside and keep warm.Meanwhile, lightly spray a non-stick fry pan with olive oil and place on a medium to high heat. Season the steak with a pinch of salt, pepper and fresh thyme, and cook for 2-3 minutes each side, or until cooked to your liking. Remove from the pan, spread evenly with the mustard and leave in a warm area to rest for a few minutes.Place the same pan back onto the heat, add the garlic and mushrooms and cook for 2 minutes. Stir in the reserved stock from the vegetables and continue to cook for a further 2-3 minutes, or until the mushrooms are cooked and the stock has reduced. Remove from the heat and keep warm.Bring a small pot of water up to the boil, and cook the beans for 2-3 minutes.Drain the green beans and place onto serving plates with the cauliflower puree and top with the cooked mustard steak. Pour over the mushrooms and serve.