Bring a medium saucepan of salted water to the boil for the pasta. Cut the chicken into 2cm chunks. Coarsely grate the cheese.
Step 2 — Cook pasta and peas
Cook the pasta in the pan of boiling water for 5 mins. Add the peas and cook for a further 2 mins or until the peas are tender and the pasta is al dente. Drain the pasta and peas, reserving the pan.
Step 3 — Cook chicken
Meanwhile, preheat the grill to high. Heat 2 tsp olive oil in a medium frypan over high heat. Cook the chicken, stirring, for 3-4 mins until browned and cooked through. Remove the chicken from the pan and season with salt and pepper.
Step 4 — Make sauce
Combine 150ml milk and 125ml (½ cup) water in a jug. Melt 20g butter in the reserved saucepan over medium heat. Add 1½ tbs plain flour and cook, stirring, for 2 mins or until foaming. Gradually add the milk mixture, stirring constantly to prevent lumps forming (see Kitchen 101). Bring to a simmer and cook, stirring, for a further 4 mins or until thickened.
Step 5 — Grill and serve up
Add the pasta and peas, barbecue seasoning, chicken and any resting juices to the sauce and stir to combine. Remove the pan from the heat. Taste, then season with salt and pepper. Transfer to a 1L (4 cup) baking dish and scatter over the cheese. Grill for 3-5 mins until the cheese is melted. Divide the chicken mac 'n' cheese among plates and enjoy!
Step 6 — Kitchen 101
The trick to achieving a silky smooth sauce is to add the liquid gradually while stirring constantly. Add a little liquid, stir until smooth, then add a little more and repeat the process until fully combined.