1. Fill your kettle and boil it. Halve the cherry vine tomatoes and set them aside. When the kettle has boiled, pour the water into a large pan (topping it up with extra water, if necessary) and add a pinch of salt. Cover and bring back to the boil. When the water is boiling, add the pasta and simmer for 8 mins till the pasta is tender but still has some bite to it. Drain the pasta and set aside.
2. While the pasta simmers, melt the butter in a medium pan over a medium heat, then stir in 2 tbsp plain flour. Cook, stirring, for 2 mins till the flour and butter are smoothly combined. This is your roux.
3. Add a splash of milk to the roux and cook, stirring, till the milk is fully incorporated. Repeat, slowly adding the milk, till it’s all combined into a sauce. Drop the bay leaf into the pan. Turn the heat up a little and bring the sauce to a simmer. Cook for 5-6 mins, stirring once or twice, till the sauce starts to thicken.
4. While the sauce simmers, grate the cheddar. Set your grill to high.
5. When the sauce has thickened a little, add three quarters of the cheddar and 1 tsp mustard. Stir till the cheese has melted. Take the sauce off the heat and grate in a quarter of the nutmeg. Stir, then taste the sauce and add a little more mustard, nutmeg or a pinch of salt or pepper if you think it needs it. Fish out the bay leaf and discard it.
6. Stir the baby leaf spinach into the sauce to wilt it. Tip the pasta into the pan and stir to mix. Spoon half the pasta into an ovenproof dish and dot with half the tomatoes. Top with the remaining pasta, sauce and tomatoes.
7. Scatter the remaining cheese over the top of the pasta and grate over a little nutmeg. Slide under the grill and cook for 5-8 mins till the top is golden and bubbling.
8. Let the pasta gratin cool for a few mins, then divide between a couple of warm bowls or deep plates and serve.