Preheat the oven to 180 degrees.
Prepare the sauce by heating the olive oil in a large saucepan over a medium-high heat. Add the garlic and stir well, cooking for 1 minute. Add the passata to the saucepan and bring to a simmer. Stir in the sugar and season with the oregano, marjoram and salt and pepper.
Simmer for 8-10 minutes, stirring occasionally until the sauce has thickened, then remove from the heat and reserve to one side.
Heat a large saucepan over a medium heat and wilt the spinach in the olive oil in batches. Transfer the wilted spinach to a colander and press it through the colander to eliminate any excess water.
Butter a 30cm x 22cm x 7cm ceramic baking dish. Combine the cooked spinach with the ricotta, Parmesan, egg and salt and pepper, mixing well to incorporate all the ingredients into one another.
Spoon this mixture into the cannelloni tubes, making sure they are filled well. Spoon some of the tomato sauce onto the base of the baking dish then arrange the filled cannelloni side-by-side in rows, making sure they are packed in tightly. Sprinkle the mozzarella on top then spoon the sauce down the centre of the dish, leaving a border on either side.
Bake for 25-30 minutes and remove when the cheese is melted and bubbling, the pasta is cooked, and the filling is hot. Allow it to sit for a few minutes before serving.
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