Put the bay leaves, leek and milk in a pan, and gently heat until steaming but just below boiling point. Take off the heat and set aside to infuse for 1 hr.
Drain the milk and discard the leek and bay. Wipe out the pan, then add the butter, flour and mustard powder. Heat, stirring, until the butter has melted, then cook to a paste for 1 min. Splash in some milk gradually and stir until smooth and completely incorporated. Add the rest of the milk, a little at a time at first, then gradually add more as the mixture gets saucy. (If you get any lumps, give it a vigorous whisk to remove them.) Heat, stirring all the time and scraping the bottom of the pan so it doesn’t catch, until the sauce is bubbling and nicely thickened. Remove from the heat and stir in 200g of the cheddar and 25g of the parmesan. Season really well – keep tasting, as it will need a generous amount of salt.
Cook the macaroni following pack instructions, until it is almost tender but still has a bit of chew to it. Drain well, then stir into the cheese sauce with three-quarters of the spring onions. Scrape into a large ovenproof dish or several individual ones.
Scatter the remaining cheddar over the macaroni, then alternate between draping over pancetta slices (bunching them up a bit) and scattering with the bread cubes and remaining spring onions. (If you keep swapping between ingredients you’ll get a nice mix of everything on the top.) Sprinkle with the remaining parmesan and add a very little drizzle of oil – holding your thumb over the bottle so just a trickle comes out. Can now be cooled, covered and chilled for up to 1 day, or frozen for up to 1 month.
Heat oven to 180C/160C fan/gas 4. Bake for 40 mins until bubbling, and crunchy and golden on top (if using individual dishes, check after 30 mins).
