1. Cook bacon
Bring a medium saucepan of salted water to the boil for the pasta. Coarsely break the cheese into small chunks. Put the bacon and a small knob of butter in a cold medium saucepan over medium-high heat. Cook, stirring occasionally, for 5 mins or until crisp.
2. Cook pasta and peas
Meanwhile, 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. Reserve 60ml (¼ cup) cooking water, then drain the pasta and peas and return to the pan.
3. Start sauce
Add 1½ tsp plain flour to the bacon and cook, stirring, for 1 min or until golden. Gradually add 60ml (¼ cup) milk and 60ml (¼ cup) water and whisk until smooth.
4. Finish sauce
Add the cheese to the bacon mixture and bring to a simmer. Reduce the heat to low and cook, stirring, for 2 mins or until the cheese is melted and the sauce is slightly thickened. Add the pasta, peas and spinach to the bacon sauce and stir to combine.
5. Serve up
Gradually add the reserved cooking water, stirring constantly, until the sauce coats the pasta and looks slightly glossy (see Kitchen 101). Taste, then season with salt and pepper. Divide the mac 'n' cheese among bowls and enjoy!
6. Kitchen 101
Using a little pasta cooking water helps emulsify the sauce so that it binds and coats the pasta. The sauce will thin out at first, but then will thicken as the starchy water is absorbed.