Step 1

To make vegetable sauce, combine water, onion and garlic in a medium saucepan. Bring to a simmer over medium heat. Reduce heat to low, cover and cook, stirring occasionally, for 5 minutes or until the onion is soft.

Step 2

Add tomatoes, celery, tomato paste and sugar. Bring to a simmer over medium-high heat. Reduce heat to medium-low, cover and cook for 10 minutes. Stir in the mushrooms and cook, covered, stirring often, for a further 5 minutes or until the mushrooms are tender. Stir in the parsley. Taste and season with salt and pepper. (You should have about 1.5L/6 cups sauce.)

Step 3

Preheat oven to 190°C. Brush a 6cm-deep, 22 x 28cm ovenproof dish with the oil to lightly grease.

Step 4

Spread just under a third of the vegetable sauce over base of greased dish. Cover with a single layer of lasagne sheets, trimming to fit. Spread spinach over. Top with half the sweet potato. Spoon over 125ml (1/2 cup) of vegetable sauce. Cover with lasagne sheets, then the remaining sweet potato. Spread with remaining vegetable sauce. Cover with lasagne sheets.

Step 5

Combine the ricotta, mozzarella and chopped oregano in a medium bowl, and then spread evenly over the top of the lasagne sheets.

Step 6

Cover with foil and bake in preheated oven for 40 minutes. Remove foil and bake for a further 15 minutes or until the pasta and sweet potato are tender when tested with a skewer. Remove from oven and set aside for 10 minutes. Sprinkle with extra oregano leaves. Serve with the lettuce salad.