Trim the baby broccoli and halve any thick stems lengthwise. Halve the leek lengthwise and rinse under running water to remove any hidden dirt. Thinly slice the leek, discarding any dark green stem. Thinly slice the garlic. Thinly shred the cabbage. Shred the carrot using a julienne peeler or coarsely grate with a box grater.
Step 2 — Prep chicken
Crumble 1 stock cube* into a heatproof jug, add 250ml (1 cup) boiling water and stir to dissolve. Put the chicken flat on a board, put your hand on top and halve horizontally into fillets. Season the chicken well with salt and pepper.
Step 3 — Start cooking chicken
Heat 1 tbs olive oil in a medium deep frypan over medium-high heat. Cook the chicken for 1-2 mins each side until browned (the chicken won't be cooked through). Remove the chicken from the pan leaving any oil in the pan.
Step 4 — Make sauce
Reduce the heat to medium. Add 2 tsp flour to the oil in the pan and cook, stirring, for 2 mins or until golden. Gradually add the stock, stirring constantly to prevent lumps forming. Whisk in 1 tsp wholegrain mustard and 1 tsp honey. Taste, then season with salt and pepper.
Step 5 — Finish chicken
Return the chicken and any resting juices to the sauce and bring to a simmer. Put the baby broccoli on top of the chicken and cook, partially covered, for 5-6 mins until the chicken is cooked through and the baby broccoli is tender.
Step 6 — Cook vegetables
Meanwhile, melt 20g butter in a medium frypan over medium-high heat. Cook the leek, garlic and a pinch of chilli flakes*, stirring, for 2-3 mins until softened. Add cabbage and carrot and cook, stirring, for a further 2-3 mins until tender. Season with salt and pepper. Divide chicken, baby broccoli, sauce and vegetables among plates to serve.