Start by heating olive oil in a pan over medium heat. Once the oil is shimmering, add in the chopped shallot and minced garlic. Cook for a few minutes until soft and fragrant, being careful not to let them burn, as skipping this step can lead to a bland sauce.
Pour in the passata and sprinkle in salt and pepper. Let the sauce simmer gently while preparing the filling. Watch for it to thicken and become vibrant. If it bubbles too much, reduce the heat.
In a mixing bowl, combine the ricotta cheese, cooked and squeezed dry spinach, beaten egg, grated parmesan, nutmeg, and salt and pepper. The mixture should have bright colors and an amazing aroma. Ensure there is no excess moisture in the spinach, as too much liquid will affect the filling.
Using a piping bag, carefully fill each cannelloni tube with the spinach and ricotta mixture. Be sure to leave a little space at both ends to prevent bursting during baking.
Spread a layer of your prepared sauce on the bottom of the baking dish to prevent the cannelloni from sticking and to keep the dish moist.
Place the filled cannelloni side by side in the baking dish over the sauce, then pour the remaining sauce over the top to coat them well. Ensure they are covered completely to avoid drying out.
Sprinkle the sliced mozzarella and extra grated parmesan on top of the cannelloni. The cheese will melt while it bakes, creating a delicious crust. Do not skimp on the cheese for the best flavor.
Place the dish into the oven and bake until bubbly and golden on top. The aroma will fill your kitchen! Monitor closely to avoid burnt edges.