Preheat the oven to 180C.
Put the mushrooms and courgettes into your food processor. Pulse to blend until they are all very roughly chopped. (Into about 1cm - 2cm pieces.)
Now add the walnuts, passata, garlic powder, oregano and pesto. Pulse again to chop the walnuts a little, reduce the size of the vegetable pieces and combine everything. (The vegetable pieces should resemble the texture of mince, without being so fine that it has disintegrated.)
In a separate bowl mix the ricotta with the lemon zest and plenty of salt and pepper.
In an ovenproof roasting dish (approx 25cm x 20cm) spoon in about a THIRD of the vegetable mixture. Top with a THIRD of the pasta sheets, and a THIRD of the ricotta mixture and a THIRD of the mozzarella.
Repeat to create two more layers, finishing on a layer of mozzarella.
Bake for 50 - 70 minutes or until the pasta is cooked through (poke a knife in to test) and the cheese is golden and bubbling.
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