Before you start, preheat the oven to 220° C (or 200° C fan bake). Bring a medium pot of salted water to the boil.
Prepare and cook roasties - prepare potatoes and toss with oil on 1-2 lined oven trays. Season with salt and pepper and roast for 25-30 minutes, until golden and tender.
Prepare leek and chicken - thinly slice leek, mince or finely chop garlic and zest lemon. Pat chicken dry and cut into 4-5 pieces. Toss chicken in a medium with lemon zest, salt, pepper, and flour.
Cook chicken - heat oil in a large fry pan on medium-high heat. Shake off excess flour from chicken (reserving leftover flour) and cook chicken for 3-5 minutes each side until nearly cooked through. Set aside, covered to keep warm.
Cook leeks - return pan to medium heat and add butter, leek and garlic, Cook for about 6 minutes until softened. Add reserved flour from chicken and cook for about 30 seconds. Add stock, mustard, and salt and bring to a simmer and cook for 2 minutes. Add cream and cook for a further 2 minutes until thickened slightly. Season to taste with salt and pepper.
Finish leeks - add cooked chicken to pan with leeks. Turn chicken a few times to coat in sauce and cook a further 1-2 minutes until chicken is warmed through.
Prepare and cook veggies - prepare broccoli and carrot. Cook broccoli and carrot in a pot of boiling water for about 5 minutes until tender. Drain and toss with lemon juice.
Serve chicken and leeks with veggies on the side.