Making the veg:
- Heat your oven to 220℃ / fan 200℃. Place the peppers, courgette and aubergine in a large roasting tin. Drizzle with extra-virgin olive oil and season well with sea salt and freshly-ground black pepper. Add the Italian herbs and garlic granules and toss to combine.
- Arrange the vegetables in a single layer; it’s alright if there’s some overlap. Roast for 20 mins, tossing once halfway through. Set aside.
- MAKE AHEAD: the vegetables can be prepared up to 3 days ahead; cool, place in an airtight container and refrigerate.
Making the sauce:
- Heat a splash of extra-virgin olive oil in a saucepan over medium heat. Add the onion, carrot and celery and season with sea salt and freshly-ground black pepper. Cook, stirring occasionally, until softened, 10 to 12 minutes. You want to gently cook the veg, not brown it.
- Add the garlic and rosemary and cook for 1 minute, stirring constantly, then add the tomato purée and cook, stirring frequently, until it begins to darken, 2 to 3 mins.
- Add the tinned tomatoes, water and sugar (if using) and season generously with sea salt and freshly-ground black pepper. Stir to combine, turn the heat up to medium-high and bring the sauce to a simmer.
- Reduce the heat to low. Simmer the sauce covered for 10 minutes, then simmer uncovered for a further 10 minutes. The sauce will thicken but still be a little looser than a traditional tomato sauce; the extra water content compensates for not pre-boiling the lasagna sheets.
- Remove the saucepan from the heat. Add the fresh basil and roasted vegetables and stir to combine. Taste and season with sea salt and freshly-ground black pepper. You want the sauce to be a little saltier than a standard pasta sauce to compensate for the fact that we are not pre-boiling the lasagna sheets in salted water.
- MAKE AHEAD: the sauce can be prepared up to 3 days ahead; cool, place in an airtight container and refrigerate. Alternatively, it can be stored in the freezer for up to 3 months.
Making the béchamel:
- While the sauce is cooking, gently bring the milk to a simmer in a saucepan over medium heat, stirring occasionally. Once it begins to bubble, remove the saucepan from the heat. Transfer the hot milk to a heatproof jug (it facilitates pouring).
- Melt the butter in a large saucepan over medium heat. Whisk in the flour and cook for 3 mins, whisking continuously.
- Quickly but carefully whisk in the hot milk and increase the heat to medium-high. Bring the béchamel to a simmer, whisking often. It will quickly begin to thicken. Once it barely starts bubbling—a few small bubbles coming up to the surface—reduce the heat to low and cook, whisking often, until thick and velvety-smooth, around 3 to 5 mins.
- Remove the saucepan from the heat and whisk in the parmigiano reggiano, sea salt, freshly-ground black pepper and nutmeg.
- MAKE AHEAD: the béchamel can be prepared the day before; cool, cover and refrigerate. Gently reheat in a saucepan to loosen before use.
Assembling the lasagna:
- Heat the oven to 180℃/fan 160℃. To layer the lasagna: 1) Spread 2 to 3 ladlefuls of the veggie-tomato sauce on the base of a tall, large roasting tin. 2) Cover with a single layer of lasagna sheets; you will likely need to cut some of the sheets to fill all the gaps. 3) Top with 2 to 3 ladlefuls of the cheesy béchamel, spreading it over the lasagna sheets in an even layer using the back of a spoon. 4) Repeat the layers—veggie-tomato sauce, lasagna sheets, béchamel—until you get to the top of the dish. Top the final layer with the grated mozzarella.
- Place the lasagna on a baking sheet (to catch spills) and bake for 40 to 45 mins, or until browned on top and bubbling at the edges.
- Let it rest for 10 mins before serving. This allows the layers to gel together and helps avoid a sloppy mess when serving.
- The lasagna can be stored in the fridge for up to 4 days. Cool, place in an airtight container and refrigerate. To reheat, heat in microwave on high until piping hot.
- The lasagna can also be stored in the freezer for up to 1 month, either baked or unbaked. Baked: cool, place in airtight, freezer-safe containers and freeze. To reheat, thaw in the fridge overnight then reheat in the microwave or oven. Unbaked: assemble the lasagna, cool, cover and freeze. To bake, thaw in the fridge overnight then bake at 180℃ /fan 160℃ for 1 hour or until browned on top and bubbling at the edges.