Heat your oven to 200°C/Fan 180°C/Gas 6. Put a large pan of salted water on to boil. Trim the roots and top 3cm off the leek. Slice a cross into the top to open it out a little. Rinse out any grit. Finely slice it. Peel and crush or grate the garlic.
When the water in the pan is boiling, add the pasta. Simmer for 5 mins, then drain.
While the pasta simmers, set a pan over a medium heat. Add the leek, 2 tbsp water and a pinch of salt and pepper. Cook, stirring now and then, for 5 mins till the leek starts to soften. Stir in the garlic. Cook for 1 min, then scoop the leek and garlic out onto a plate or bowl.
Add the butter to the empty pan and melt it over a medium heat, then stir in 2 tbsp flour. Cook and stir for 2 mins to make a smooth paste.
Measure out 400ml of the milk. Add a small splash of milk to the flour and butter roux and whisk to combine – it should make a smooth, thick paste. Add another splash and whisk again. Keep going till you’ve smoothly incorporated all the milk with the roux. Turn up the heat a little and bring the mix to a boil, then turn it down and simmer for 8-10 mins till the sauce has slightly thickened. Give the mix a stir every so often.
While the white sauce simmers, coarsely grate half the cheddar. The sauce should have thickened a little by now. Take it off the heat and stir in the cheddar and baby leaf spinach. Stir till the cheese melts and the spinach wilts. Taste and add a pinch more salt or pepper if needed.
Arrange the drained pasta in an ovenproof dish and stir in the cooked leeks and fish pie mix. Pour the cheese and spinach sauce over the top. Scatter over the breadcrumbs. Slide into the oven and bake for 30 mins till bubbling.
While the pasta bake cooks, toss the salad leaves with 2 tsp olive oil and a pinch of salt and pepper. Serve the pasta bake in warm bowls with the salad on the side.