Preheat the oven to 220C, fan-forced. Line an oven tray with baking paper. Cut the unpeeled sweet potatoes into 5mm-thick rounds. Thinly slice the zucchini into rounds. Thinly slice the onions. Coarsely grate the cheese. Put the breadcrumbs and 2 tsp olive oil in a bowl and, using clean hands, rub together until well combined. Set aside.
Step 2 — Bake sweet potato
Put the sweet potato on the lined tray. Drizzle with 1 tbs olive oil, scatter with half the Tuscan seasoning, lightly season with salt and pepper and toss to combine. Bake for 15 mins or until golden and tender. Reduce the oven to 200C.
Step 3 — Make pesto sauce
Meanwhile, heat 2 tsp olive oil in a medium frypan over medium heat. Cook the onion, stirring, for 3-4 mins until softened. Add the tomato passata, pesto, remaining Tuscan seasoning and 200ml hot water and bring to the boil. Reduce heat to medium. Cook for 7-8 mins until slightly reduced. Taste, then season with salt and pepper. Remove from the heat.
Step 4 — Make bechamel sauce
Melt 20g butter in a medium saucepan over medium heat. Add 35g (¼ cup) plain flour and cook, stirring, for 2 mins or until golden. Gradually add 300ml hot water and 125ml (½ cup) milk, stirring constantly to prevent lumps forming. Bring to a simmer and cook, stirring, for 2-3 mins until thickened. Taste, then season with salt and pepper. Remove the pan from the heat.
Step 5 — Assemble lasagne
Spread one-third of the pesto sauce over the base of a 2L (8 cup) baking dish. Layer one-third of the sweet potato, one-third of the zucchini and one-third of the spinach over the sauce. Top with 2 lasagne sheets, trimming to fit, if necessary. Spread one-third of the bechamel sauce over the lasagne sheets.
Step 6 — Bake lasagne and serve up
Repeat the layers two more times, with the remaining pesto sauce, sweet potato, zucchini, spinach, lasagne sheets* and bechamel sauce. Scatter over the cheese and breadcrumbs. Cover the lasagne with foil and bake for 15 mins. Uncover and bake for a further 30 mins or until golden and cooked through. Remove from oven and stand for 5 mins. Serve lasagne at the table and enjoy!