Mix tomato purée, chopped tomatoes, water, vegetable stock cube, oregano, sugar, salt and pepper together in a bowl
Preheat the oven to 190°C and grab a 9 inch baking tray
Dry-fry the mushrooms in 3 batches in a large frying pan on medium heat for 2 minutes without moving, then turn over, season with salt and pepper, and fry for another 1 minute. Scrape each batch into a bowl
Using the same pan on medium heat, add butter and 2 tbsp olive oil followed by the leek and fry for 2 minutes
Add garlic and herbs and cook for 3 minutes
Mix mushrooms back into the pan and check seasoning
Add lasagna sheets to soak in a bowl of boiled water for 2-3 minutes
Cover the bottom of the tray with tomato sauce, grate over cheese, and top with a lasagna layer
Add more tomato sauce, half the mushroom mix, half the mozzarella, grated cheese, and season
Repeat the layering process
Add double cream and 1 tbsp olive oil on top
Place into the oven for 25-30 minutes or until bubbling and golden
Rest for 10 minutes before slicing
