Step 1 Preheat oven to 160°C. Spray a medium non-stick frying pan with olive oil and set over high heat. Add leek and cook, stirring occasionally, for 3–4 minutes, or until soft. Add mushrooms to pan and cook, stirring occasionally, for 5 minutes, or until golden. Add garlic and baby spinach to pan; cook, stirring, for 1 more minute. Remove frying pan from heat and set aside to cool.
Step 2 Place cool mushroom mixture in a medium bowl with reserved roast pumpkin and ricotta; stir to combine filling. Spoon 2/3 cup of the pasta sauce into a 2-litre (8-cup)-capacity baking dish.
Step 3 Cut each lasagne sheet in half crossways. Spoon some of the filling along centre of each half sheet and roll to enclose filling (this makes 8 cannelloni).
Step 4 Place cannelloni, seam-side down, in prepared baking dish; cover with remaining pasta sauce and sprinkle with parmesan (if using). Bake for 25 minutes, or until cannelloni are cooked.
Step 5 Place 2 cannelloni on each serving plate and serve with extra baby spinach.