Stir in the oyster sauce and cook for 2–3 minutes. Add about half of the reserved broccoli cooking water and crumble in half of the chicken stock cube. Bring to a simmer and then add the sesame oil.
Pour in all of the chicken stock from the poaching pan. Add the broccoli and simmer, breaking down the broccoli with the back of a spoon. As it cooks, mash the broccoli into a very soft sauce.
Once the pasta is cooked and strained, add the contents of the saucepan to the chicken and broccoli sauce. Stir well to combine the garlicky cheesy sauce with the pasta and broccoli florets.
then add the chicken to the sauce. Season with salt and freshly ground pepper. Butter an ovenproof lasagne dish, put a layer of broccoli on the base, scatter the mushrooms on top and cover with ...