Preheat the oven to 180 degrees Celsius.
Put the sausages on a baking tray and put in the oven for 15 minutes, then add the mushrooms to the tray and cook for another 15 minutes. Remove from the oven (leave the oven on) and allow to cool slightly before cutting the sausages into 1-2cm slices.
Get a large ovenproof dish. Add 2 tablespoons of the passata to the bottom of the dish and spread it evenly until it covers the base. Top with 1/3 of the lasagne sheets, covering it in an even layer.
Top this with an even layer of half the sausages and half the mushrooms. Use a spoon to dot half the ricotta evenly over the top, then pour over half of the remaining passata. Sprinkle with a pinch of salt and pepper, and half of the oregano.
Layer over another 1/3 of the lasagne sheets, then repeat with the remaining sausage, mushrooms, ricotta, passata and oregano. Top with the remaining lasagne sheets.
In a separate bowl, whisk together the creme fraiche, Parmesan and egg. Pour this over the lasagne, ensuring it covers the entirety of the top layer. Sprinkle over the cheddar cheese, then bake in the oven for 30 minutes.