Heat olive oil or butter in a skillet over medium heat. Add the onion and garlic, cooking until fragrant and softened, about 2–3 minutes.
Add the chopped shrimp and cook until just pink, about 2–3 minutes. Stir in the crab meat and season with salt and pepper to taste. If using spinach, stir it in now and cook for another minute until just wilted.
Remove from heat and let the mixture cool slightly.
In a large bowl, combine the seafood mixture with the ricotta or cream cheese and half of the Parmesan cheese. Stir until everything is evenly incorporated into a creamy, flavorful filling. Set aside.
Step 2: Make the Lemon Cream Sauce
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk constantly for about 1 minute to form a smooth paste (roux).
Gradually whisk in the milk or cream, pouring a little at a time to prevent lumps. Continue whisking until the sauce thickens and coats the back of a spoon, about 5 minutes.
Stir in the lemon zest and juice, then season with salt and pepper to taste. Remove from heat — the sauce should be silky and slightly tangy with a bright lemon aroma.
Step 3: Assemble the Cannelloni
Preheat your oven to 180°C (350°F). Lightly grease a large baking dish.
Using a piping bag or small spoon, fill each cannelloni tube with the shrimp and crab mixture. Arrange the filled cannelloni in a single layer in the prepared dish.
Pour the lemon cream sauce evenly over the stuffed cannelloni, making sure all the pasta is covered.
Sprinkle the top with the remaining Parmesan cheese and all of the mozzarella cheese.
Step 4: Bake to Perfection
Place the dish in the preheated oven and bake for 25–30 minutes, or until the top is bubbly, golden, and slightly crisp around the edges.
Let the cannelloni rest for about 5 minutes before serving — this helps the filling set and makes slicing easier.
