1. Preheat your oven to 180°C/Fan 160°C/Gas 4. Put a large pan of salted water on to boil.
2. Get the veg prepped^
Trim the dry end off the broccoli stalk, then chop the broccoli into small florets and the stalk into bite-sized pieces. Set aside. Trim the roots and top 3cm off the leeks. Slice a cross in the tops of the leeks, open them out and rinse out any grit. Finely slice the leeks. Halve the cherry tomatoes.
3. Start the cheese sauce:
Put a medium pan on a medium heat and add the butter. Swirl it around to melt the butter, then add the leeks and the bay leaf with a pinch of salt and pepper. Fry, stirring often, for 5 mins till the leeks look glossy and are lightly browned.
4. Add 2 tbsp flour to the leeks and cook, stirring, for 2 mins. Measure out 800ml of the milk. Add a splash of the milk and stir it into the leeks and flour till combined, then carry on adding splashes of milk, stirring well between each addition, till all 800ml has been added. This should take around 5-6 mins. This is your white sauce.
5. Once you’ve added all the milk, let the white sauce come up to a bubbling simmer. Don’t let it get too hot or it will boil over. As soon as it’s bubbling, turn the heat down to low and simmer for 3-4 mins, stirring often, till the sauce starts to thicken a little.
6. Cook the pasta & broccoli:
When the pan of water is boiling, add the penne. Simmer for 6 mins, then add the broccoli and simmer for 2-3 mins till the penne is tender but still has some bite, and the broccoli is bright green. Drain and set aside.
7. Finish the cheese sauce:
While everything simmers, coarsely grate the cheddar. Once the white sauce has started to thicken (if you stir it with a wooden spoon, you should just see some trails in the sauce), take it off the heat. Fish out the bay leaf. Add 3/4 of the cheddar and grate in plenty of nutmeg. Stir in the tomatoes and spinach. Taste the sauce and add more salt, pepper or nutmeg if you think it needs it.
8. Into the oven:
Tip the penne and broccoli into a large ovenproof dish. Pour over the cheesy white sauce and stir gently to mix. Sprinkle over the breadcrumbs and top with the last of the grated cheddar. Slide into the oven and bake for 20-25 mins till golden and bubbling. Serve in warm bowls
9. Get Ahead Tip
You can assemble this pasta bake the night before you want to cook it. Make the pasta bake, topping it with the breadcrumbs and cheddar, then store in the fridge overnight. When you're ready to cook, take the dish out of the fridge and loosely cover the top with foil. The pasta bake will take around 40 mins to bake from the fridge, so covering the top stops it burning. Remove the foil for the final 15 mins cooking.
10. Eat Me, Keep Me Tip
The cooked pasta bake will keep in the fridge for up to 3 days or you can freeze it for up to 3 months. Divide it into individual tubs, cool then seal and chill or freeze. Defrost completely before reheating in the microwave, or warm in the oven in a dish loosely covered with foil till piping hot all the way through.