1. Get set up:
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 roots and top 3cm off the leeks, then halve them, rinse out any grit and finely slice them. Rinse the mushrooms, then tear them into bitesized chunks.
3. Start the cheese sauce:
Put a medium pan on a medium heat and add 1 tbsp garlic oil and butter. Swirl to melt the butter, then add the leeks and mushrooms with a pinch of salt and pepper. Fry, stirring often, for 6-7 mins till the veg look glossy and are lightly browned.
4. Add 2 tbsp flour to the veg and cook, stirring, for 2 mins. Measure out 800ml of the milk. Add a splash of milk and stir it into the veg 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 3-4 mins. This is your white sauce.
5. Once you’ve added all the milk, let the sauce come up to a bubbling simmer. Don’t let it get too hot or it will boil over. When it’s bubbling, turn the heat down to low and simmer for 5-8 mins, stirring often so it doesn’t stick, till the sauce starts to thicken.
6. Cook the pasta:
When the pan of water is boiling, add the gnocchetti. Simmer for 8 mins till the pasta is tender but still has some bite. Drain and set aside.l
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. Add ¾ of the cheddar and stir to melt it. Add the spinach and stir to wilt it. Taste the sauce and add more salt or pepper if you think it needs it.
8. Into the oven:
Tip the pasta into a large ovenproof dish. Pour over the white sauce and stir gently to mix. Top with the remaining cheddar. Slide into the oven and bake for 20-25 mins till golden and bubbling. Serve in warm bowls, with a little pesto spooned over each.
9. Get Ahead:
You can assemble this pasta bake the night before you want to cook it. Make the pasta bake, topping it with the 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:
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.