To make the bechamel sauce, combine the flour and butter in a medium saucepan over medium heat. Cook, stirring for 2 minutes, until the mixture forms a roux. Gradually add the milk, stirring well to avoid any lumps, to a smooth sauce. Cook, stirring occasionally until lightly thickened, then stir through the chicken stock powder (or salt).Bring a large saucepan of salted water to the boil over high heat, then cook the pasta according to the package directions, checking 2 minutes before the printed time on the packet.Heat a large, heavy-based (oven-proof) saucepan over medium heat and preheat the oven to 180˚C. Add the oil, onion, mushrooms and garlic to the heavy-based saucepan and cook, stirring, for 6 minutes, or until softened. Stir through the chicken and cook until lightly browned. Scoop out a generous splash of pasta cooking water to the pan with the chicken and scrape up any browned bits from the base of the pan.Scoop up 1 cup of pasta cooking water, then drain and transfer the cooked penne to the pan with the chicken with the baby spinach with a generous splash of reserved pasta cooking water and stir until the spinach has wilted. Stir through half the bechamel sauce.Flatten the pasta in the pan, then pour over the remaining bechamel sauce, smooth out and then sprinkle with the grated cheese. Bake for 15 minutes, then change the oven setting to grill and cook for 10 minutes further, or until the cheese is golden and bubbly.Remove the Japanese chicken and pasta grain from the oven and allow to stand for 15 minutes before serving.Photography by Jiwon Kim.Want more from The Cook Up? Stream all the seasons here and for free at