1. Render the lardons on bacon down quite slowly until crispy. This takes about 15 minutes.
2. Then soften the sliced leeks in the bacon fat, add the garlic after 5 minutes. You want to get them nice and soft, so that the inner rings pop out easily. It'll take about 15 more minutes. Add a knob of butter to help the leeks soften. (You could fry off the bacon and leeks at the same time in separate pans, saving time but not washing up.)
3. Meanwhile cook the pasta in a large pan of boiling water until al dente. Drain but retain a mugful of pasta water!
4. Make the sauce in the microwave. Put the flour, butter and milk in a large microwaveable jug or bowl. Set on high for ten minutes, whisking every minute or so. It will suddenly thicken, at that point turn the heat to medium and cook out for two more minutes. Add the cheese and mustard.
5. Mix the bacon and leeks, cheese sauce and pasta and pour into a baking dish. Add a small splash of pasta water - this prevents it from being too claggy (and makes it more saucy - remember the pasta soaks up sauce in the oven). Top with breadcrumbs and grate fresh parmesan on top.
6. Bake for 20 minutes, or until brown and bubbling.