Before you start, preheat oven to 220°C (or 200°C fan bake). Bring a large pot of salted water to the boil. Set aside a large baking dish.
Prep potatoes, Brussels sprouts and leek. Place potatoes in pot of boiling water, bring back to the boil and cook for 5 minutes. Add Brussels sprouts and leek and cook for a further 5 minutes, until just tender. Drain and transfer to reserved baking dish.
Prep sauce. Melt butter in a medium pot on medium-high heat. Add flour, stir and cook for 30 seconds. Pour in half the milk and whisk to combine, ensuring there are no lumps, then add remaining milk and stock. Add half the grated cheese and salt and whisk to combine.
Pour sauce over veggies, distributing evenly. Top with remaining cheese. Bake for 15-20 minutes, until golden on top.
Pat chicken dry and cut into steaks. Place your hand flat on top of the chicken and slice through horizontally. Season with salt and pepper.
Heat oil in a large fry-pan on medium-high heat. Cook chicken for 3-5 minutes each side (depending on thickness), or until cooked through. Rest, covered. Slice thinly and toss with any resting juices, if desired.
While chicken cooks, prep broccoli and place on a lined oven tray with oil. Season with salt and pepper and cook on rack below potatoes for an initial 10 minutes. After 10 minutes, sprinkle almonds over broccoli and cook a further 4-5 minutes, until golden and broccoli is cooked through. Season with salt to taste.
Serve chicken with gratin and broccoli.